Tests: Use more appropriate assertions in get_themes() tests.

This replaces instances of `assertTrue( is_file( ... ) )` followed by `assertTrue( is_readable( ... ) )` with `assertFileIsReadable()` to use native PHPUnit functionality.

The `assertFileIsReadable()` method was introduced in PHPUnit 5.6. As the minimum supported PHPUnit version has been raised to PHPUnit 5.7.21, it can now be used.

Follow-up to [51543], [51574].

Props jrf.
See #53363.

git-svn-id: https://develop.svn.wordpress.org/trunk@51579 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tests/phpunit/tests/theme.php

@@ -148,25 +148,22 @@ function test_get_themes_contents() {
 			$this->assertIsArray( $theme['Template Files'] );
 			$this->assertNotEmpty( $theme['Template Files'] );
 			foreach ( $theme['Template Files'] as $file ) {
-				$this->assertTrue( is_file( $dir . $file ) );
-				$this->assertTrue( is_readable( $dir . $file ) );
+				$this->assertFileIsReadable( $dir . $file );
 			}
 
 			// CSS files should all exist.
 			$this->assertIsArray( $theme['Stylesheet Files'] );
 			$this->assertNotEmpty( $theme['Stylesheet Files'] );
 			foreach ( $theme['Stylesheet Files'] as $file ) {
-				$this->assertTrue( is_file( $dir . $file ) );
-				$this->assertTrue( is_readable( $dir . $file ) );
+				$this->assertFileIsReadable( $dir . $file );
 			}
 
 			$this->assertTrue( is_dir( $dir . $theme['Template Dir'] ) );
 			$this->assertTrue( is_dir( $dir . $theme['Stylesheet Dir'] ) );
 
 			$this->assertSame( 'publish', $theme['Status'] );
 
-			$this->assertTrue( is_file( $dir . $theme['Stylesheet Dir'] . '/' . $theme['Screenshot'] ) );
-			$this->assertTrue( is_readable( $dir . $theme['Stylesheet Dir'] . '/' . $theme['Screenshot'] ) );
+			$this->assertFileIsReadable( $dir . $theme['Stylesheet Dir'] . '/' . $theme['Screenshot'] );
 		}
 	}