Update tests to ensure MailMessage implements the fluent interface pattern (#51969)

Author: seriquynh

tests/Mail/MailMessageTest.php

@@ -26,65 +26,65 @@ protected function setUp(): void
 
     public function testFromMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->from('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getFrom()[0]);
+        $this->assertSame($this->message, $this->message->from('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getFrom()[0]);
     }
 
     public function testSenderMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->sender('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getSender());
+        $this->assertSame($this->message, $this->message->sender('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getSender());
     }
 
     public function testReturnPathMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->returnPath('[email protected]'));
-        $this->assertEquals(new Address('[email protected]'), $message->getSymfonyMessage()->getReturnPath());
+        $this->assertSame($this->message, $this->message->returnPath('[email protected]'));
+        $this->assertEquals(new Address('[email protected]'), $this->message->getSymfonyMessage()->getReturnPath());
     }
 
     public function testToMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->to('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getTo()[0]);
+        $this->assertSame($this->message, $this->message->to('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getTo()[0]);
 
-        $this->assertInstanceOf(Message::class, $message = $this->message->to(['[email protected]' => 'Bar']));
-        $this->assertEquals(new Address('[email protected]', 'Bar'), $message->getSymfonyMessage()->getTo()[0]);
+        $this->assertSame($this->message, $this->message->to(['[email protected]' => 'Bar']));
+        $this->assertEquals(new Address('[email protected]', 'Bar'), $this->message->getSymfonyMessage()->getTo()[0]);
     }
 
     public function testToMethodWithOverride()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->to('[email protected]', 'Foo', true));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getTo()[0]);
+        $this->assertSame($this->message, $this->message->to('[email protected]', 'Foo', true));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getTo()[0]);
     }
 
     public function testCcMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->cc('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getCc()[0]);
+        $this->assertSame($this->message, $this->message->cc('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getCc()[0]);
     }
 
     public function testBccMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->bcc('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getBcc()[0]);
+        $this->assertSame($this->message, $this->message->bcc('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getBcc()[0]);
     }
 
     public function testReplyToMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->replyTo('[email protected]', 'Foo'));
-        $this->assertEquals(new Address('[email protected]', 'Foo'), $message->getSymfonyMessage()->getReplyTo()[0]);
+        $this->assertSame($this->message, $this->message->replyTo('[email protected]', 'Foo'));
+        $this->assertEquals(new Address('[email protected]', 'Foo'), $this->message->getSymfonyMessage()->getReplyTo()[0]);
     }
 
     public function testSubjectMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->subject('foo'));
-        $this->assertSame('foo', $message->getSymfonyMessage()->getSubject());
+        $this->assertSame($this->message, $this->message->subject('foo'));
+        $this->assertSame('foo', $this->message->getSymfonyMessage()->getSubject());
     }
 
     public function testPriorityMethod()
     {
-        $this->assertInstanceOf(Message::class, $message = $this->message->priority(1));
-        $this->assertEquals(1, $message->getSymfonyMessage()->getPriority());
+        $this->assertSame($this->message, $this->message->priority(1));
+        $this->assertEquals(1, $this->message->getSymfonyMessage()->getPriority());
     }
 
     public function testBasicAttachment()