Let tests fail on deprecation notices

This is what we need to fix:

Testing Drupal\Tests\address\Kernel\CountryNameTokenTest

OK (2 tests, 17 assertions)

Remaining deprecation notices (2)

  2x: Special handling of system module schemas in \Drupal\KernelTests\KernelTestBase::installSchema has been deprecated in Drupal 8.7.x, remove any calls to this method that use invalid schema names. See https://www.drupal.org/node/3003360.

CountryNameTokenTest needs the fix from https://www.drupal.org/node/3041703

That will resolve:

 3x: Drupal\Tests\taxonomy\Functional\TaxonomyTestTrait trait is deprecated in Drupal 8.8.0 and will be removed before Drupal 9.0.0. Use \Drupal\Tests\taxonomy\Traits\TaxonomyTestTrait instead. See https://www.drupal.org/node/3041703.

Wheee! Glad I documented a solution to that which works for 8.7.x, too. ;)

Argh, once again, the thing we need to use isn't even available in 8.7.x. :( Ugh.

Screw this. We can be Ready For D9(tm) when 8.7.x is EOL. Until then, I have better things to do with my time than fight these uphill and frustrating battles.

(whoops, I missed text areas in #8 since the summary was empty. Stub for later

Guess I'm a glutton for punishment and I can't stop myself. ;)

Re-queued #11 on 8.8.x and 8.9.x now that #3089688-11: Make the build pass on Drupal 8.8 is fixed again.

Alas, function_exists() doesn't do exactly what I thought. is_callable() seems to work locally for what I'm trying to do here.

8.6.x had what looks like a random fail:

There was 1 error:

1) Drupal\Tests\address\Functional\Views\CountrySortTest::testSortCountryCode
mkdir(): File exists

/var/www/html/core/tests/Drupal/Tests/BrowserHtmlDebugTrait.php:141
/var/www/html/core/tests/Drupal/Tests/BrowserTestBase.php:404
/var/www/html/modules/contrib/address/tests/src/Functional/Views/CountrySortTest.php:51

Re-queued that.

Assuming it passes, I'd like to revert the change to composer.json here that's requiring core ^8.7. No need to force that until 8.6.x is actually EOL. Or really, until we start doing things that don't exist in 8.6.x. So far, it doesn't seem like that's necessary, so let's not put folks stuck on 8.6.x for other reasons out of luck.

Indeed, we're still good with 8.6.x, so I reverted that part of the composer.json change.
I also opened #3093193: Require Drupal 8.8 as a target for @see @todo comments to clean this stuff up. ;)
Pretty sure this is RTBC, but I'll queue it for a final round of testing on All The Branches(tm).

Committed #15 and pushed to 8.x-1.x.

Thanks, dww!

Per a Slack discussion with Gábor Hojtsy regarding usage of D9 tags (Drupal 9, Drupal 9 compatibility, Drupal 9 readiness, etc.), "Drupal 9 compatibility" should be used for contributed projects that need updating and "Drupal 9" was the old tag for D8 issues before the D9 branch was ready. Doing tag cleanup here based on that discussion.