ConfigEntityType "missing 'config_export" error message doesn't say what annotation is missing

ConfigEntityTypeInterface defines a getClass() method
We should just use that, which will return the specific class with the annotation issue.

One-liner

Update test coverage for this change.

Fixed test coverage.

Fixed the patch for 9.0.x.
The method TestCase::expectDeprecationMessageRegExp() has been renamed to TestCase::expectDeprecationMessageMatches().

The patched code ends with a blank line which somehow was also shown by the IDE in the #7 patch. Removed the second blank line at the end and hope this works now.

Sorry for the clutter. It took me a while to find out that the error "trailing whitespace" actually means "windows line ending" which I forgot to change in my editor after a re-installation.

+++ b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php
@@ -635,8 +635,11 @@ public function testToArraySchemaException() {
+      ->willReturn("Mock_ConfigEntityTypeInterface");
...
+    $this->expectExceptionMessage("Entity type 'Mock_ConfigEntityTypeInterface' is missing 'config_export' definition in its annotation");

The test is green, but in reality, it never throws error on specific Interface, it's against specific Class.

Let's change Mock_ConfigEntityTypeInterface to FooConfigEntity to make sense more

Addressed #15

Thanks, it's ready to me.

Committed and pushed b2519618b4 to 9.2.x and c8b93171c9 to 9.1.x. Thanks!