PHPUnit test methods can be set to be skipped by applying Skip attribute

A new PHP attribute, \Drupal\TestTools\Attribute\Skip is available to mark PHPUnit test methods to be skipped. Using the new attribute is preferred instead of calling \PHPUnit\Framework\Assert::markTestSkipped(). The presence of the attribute is checked before Drupal is bootstrapped in Kernel and Functional tests, providing a performance improvement when the test is skipped.

Before:

public function testFoo(): void {
  $this->markTestSkipped('Information about why the test is skipped.');
  // Test code.
}

public function testBar(): void {
  $this->markTestSkipped();
  // Test code.
}

After:

#[Skip('Information about why the test is skipped.')]
public function testFoo(): void {
  // Test code.
}

#[Skip]
public function testBar(): void {
  // Test code.
}