Validate that uploaded .po files are UTF8

I rebased the MR onto 11.x but there are errors to fix.

Not the most elegant solution, but does the job.

All issues should be resolved now.

Ran test-only feature

1) Drupal\Tests\locale\Functional\LocaleImportFunctionalTest::testStandalonePoFile
Behat\Mink\Exception\ResponseTextException: The text "The file is not encoded with UTF-8" was not found anywhere in the text of the current page.
/builds/issue/drupal-1630568/vendor/behat/mink/src/WebAssert.php:907
/builds/issue/drupal-1630568/vendor/behat/mink/src/WebAssert.php:293
/builds/issue/drupal-1630568/core/tests/Drupal/Tests/WebAssert.php:975
/builds/issue/drupal-1630568/core/modules/locale/tests/src/Functional/LocaleImportFunctionalTest.php:257
FAILURES!
Tests: 6, Assertions: 136, Failures: 1.

Which shows the coverage.

Believe all feedback on the MR has been addressed.

This is almost there. I left a few comments to look at.

I think other threads are solved, but still open to suggestions if the FileEncoding constraint $encoding should be list of encodings.

Sorry that I missed that this will need tests of the new constraint validator.

I added tests for FileEncodingConstraint. Instead of list I converted $encoding into an array, IMO it makes the code cleaner: no need for implode or preg_* that FileExtensionConstraint is using.

I reviewed this and it looks great.

All of the threads seem to be resolved.

I tried to run the test only job, but it wouldn't run due to:
This job could not start because it could not retrieve the needed artifacts.
Learn more about dependencies and common causes of this error.

The actual tests are passing.

Rebased and got test-only to run https://git.drupalcode.org/issue/drupal-1630568/-/jobs/2659732

Re-looking at believe all feedback has been addressed

Committed 02b324c and pushed to 11.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.