more precise non-isolated assertions

Author: staabm

tests/end-to-end/regression/5592.phpt

@@ -66,4 +66,4 @@ Test code or tested code removed exception handlers other than its own
 %sIssue5592Test.php:%i
 
 FAILURES!
-Tests: 6, Assertions: 6, Failures: 4, Risky: 4.
+Tests: 6, Assertions: 10, Failures: 4, Risky: 4.

tests/end-to-end/regression/5592/Issue5592Test.php

@@ -13,6 +13,7 @@
 use function restore_exception_handler;
 use function set_error_handler;
 use function set_exception_handler;
+use Closure;
 use PHPUnit\Framework\TestCase;
 use PHPUnit\Runner\ErrorHandler;
 use Throwable;
@@ -47,15 +48,15 @@ public function testAddedAndRemovedExceptionHandler(): void
         $previous = set_exception_handler([$this, 'addedAndRemovedExceptionHandler']);
         restore_exception_handler();
 
-        $this->assertNull($previous);
+        $this->assertInstanceOf(Closure::class, $previous);
         $this->assertTrue(true);
     }
 
     public function testAddedExceptionHandler(): void
     {
         $previous = set_exception_handler([$this, 'addedExceptionHandler']);
 
-        $this->assertNull($previous);
+        $this->assertInstanceOf(Closure::class, $previous);
         $this->assertTrue(false);
     }