Add tests for the taxonomy Views plugins

Thanks a lot for your work, looks pretty good already!
Just two small complaints:

+++ b/tests/src/Kernel/Plugin/views/argument/SearchApiTermTest.php
@@ -0,0 +1,114 @@
+namespace Drupal\tests\search_api\Kernel\Plugin\views\argument;

I think this should just be called SearchApiTermArgumentTest and live in the (existing) Drupal\tests\search_api\Kernel\Views namespace. Adding three levels of namespace with just a single class doesn’t seem sensible.
If we get enough test classes for single plugins, we can still move them to a sub-namespace later.

+++ b/tests/src/Kernel/Plugin/views/argument/SearchApiTermTest.php
@@ -0,0 +1,114 @@
+    self::assertNull($plugin->title());

I know it’s a static method, but assertions are still called dynamically (i.e., via $this->assert_()) nearly exclusively. For consistency, I’d therefore also prefer this here. (Same for all other assertions.)

I think testing this with a kernel test is sufficient, I don't think we should add an integration test for this as well.

I disagree, with this issue I actually had an integration test in mind. We now have three different taxonomy-related Views plugins, it would have seemed sensible adding a single integration test for all of them, like we have one for most other plugins in ViewsTest::testView().
Getting coverage through Kernel tests is of course also great, but since in this case we override just a single method, it’s clear that this has no chance of uncovering all problems we might run into in the future. This depends a lot on Views internals, our own traits, etc., and I think we make sure the overall functionality remains working, not just the one method we actually override in the plugin classes (two, now, with SearchApiAllTerms).

So, while I’ll surely also commit this Kernel test, I wouldn’t consider this issue fixed until we have an integration test for at least part of this functionality. It shouldn’t be too difficult, just needs a test setup that also includes taxonomy terms.

Alright, thanks!
Then just a few nit-picks on my part. If those are fine with you, I’ll commit this (and set back to “Active“ for the integration test – if I remember).

Alright, committed.
Thanks again!

Setting back to “Active” for the integration test.