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

This replaces instances of `assertTrue( is_dir( ... ) )` with `assertDirectoryExists()` to use native PHPUnit functionality.

The `assertDirectoryExists()` 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], [51579].

Props jrf.
See #53363.

git-svn-id: https://develop.svn.wordpress.org/trunk@51580 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tests/phpunit/tests/theme.php

@@ -158,8 +158,8 @@ function test_get_themes_contents() {
 				$this->assertFileIsReadable( $dir . $file );
 			}
 
-			$this->assertTrue( is_dir( $dir . $theme['Template Dir'] ) );
-			$this->assertTrue( is_dir( $dir . $theme['Stylesheet Dir'] ) );
+			$this->assertDirectoryExists( $dir . $theme['Template Dir'] );
+			$this->assertDirectoryExists( $dir . $theme['Stylesheet Dir'] );
 
 			$this->assertSame( 'publish', $theme['Status'] );