Add assertNoDuplicateIds to a functional test trait

@nnevill if you don't mind I'll help you with #5

1) I guess we don't need to check if there are any HTML IDs here assertNoDuplicateIds. So, I removed this check (like in KernelTests)
2) I've added tests (positon + negative) for assertNoDuplicateIds.

@Lendude should this not go in \Drupal\Tests\WebAssert? And be accessed in tests via $this->assertSession()-> assertNoDuplicateIds() - maybe the method name needs to change.

This is even better version.
Looks good to me. All remarks were addressed.

1. +++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
   @@ -8,6 +8,7 @@
   +use PHPUnit\Framework\AssertionFailedError;
   
   +++ b/core/tests/Drupal/Tests/BrowserTestBase.php
   @@ -7,6 +7,7 @@
   +use Drupal\Component\Render\FormattableMarkup;
   

   Unused.

2. +++ b/core/tests/Drupal/Tests/WebAssert.php
   @@ -597,4 +597,26 @@ public function pageTextContainsOnce($text) {
   +        throw new ExpectationException($message, $this->session->getDriver());
   

   Let's use sprintf() to add the ID to the message.

If. Allow tests to compare MarkupInterface objects via assertEquals() has already been taken care of in 9.x than adding patch only for point mentioned in: #11

If we need a patch that has both changes. of. earlier patch #6 and #11 Use, this. patch.

Solving failed test cases. Kindly review a new patch.

Removing unrelated changes introduced in #16 , and creating patch that has both changes as mentioned at #9 and #11. Kindly review new patch.

1. I added a Change Record. The pageContainsNoDuplicateId assert was added to WebAssert.

2. Addressed #11.2 and added a test for it.

-        throw new ExpectationException(sprintf($message), $this->session->getDriver());
+        throw new ExpectationException(sprintf('The page contains a duplicate HTML ID "%s".', $id), $this->session->getDriver());

+ $this->expectExceptionMessage('The page contains a duplicate HTML ID "page-element".');

3. I removed an unnecessary assert. It does nothing.
- $this->assert(TRUE, $message);

Unrelated random test failure.

Fixed coding standards.

I think all unrelated changes introduced in #16 is removed now , and rename from testAssertNoDuplicateIds to testPageContainsNoDuplicateId make sense also coding standards is proper now . So moving to RTBC.

There will be two the same asserts, but with a different name, assertNoDuplicateIds for kernel and pageContainsNoDuplicateId for functional... They should be standardized.

https://git.drupalcode.org/project/drupal/-/blob/9.0.x/core/tests/Drupal...
protected function assertNoDuplicateIds($message = '', $group = 'Other', $ids_to_skip = []) {

Committed 3c4c7a4 and pushed to 9.1.x. Thanks!

Not moving to 8.9/9.0 for backport. This is nice to have, but the patch doesn't result in a lot of test divergence so I think it's fine to just add it in 9.1.x

Backport to 8.9/9.0 is not necessary, but it is helpful for another issue. See #2894747-37: Views hardcodes exposed filter block form ID's which breaks AJAX when the same form is shown multiple times on one page. Instead of adding another method to core/modules/views/tests/src/Functional/Plugin/ExposedFormTest.php, we could use the new method from WebAssert.