Upgrade to phpunit 10

[uuf6429]

Motivation:

• support phpunit.dist.xml 😆
• phpunit 9 is in maintenance mode
• phpunit 10 supports php ^8.1 (and we recently switched this repo that minimum version too)
• and additionally:
  • fix deprecation Deprecate "Test" suffix for abstract test case classes sebastianbergmann/phpunit#5132 (which is now an error Abstract *Test classes cannot be found sebastianbergmann/phpunit#4979) about abstract classes ending in Test
  • update the phpunit xml config file schema

AbstractFilterTestBase and AbstractKeywordsTestBase were renamed due to the aforementioned phpunit change. I picked that name because:

• Abstract- puts the class on top of the file list and immediately gives the impression of an abstract class
• -Base is specifically the fix for the problem of ending with -Test

Alternative ideas for a name are of course welcome.

Renamed to -TestCase instead, since it feels more natural.

[acoulton]

Thanks @uuf6429 this looks generally good, a couple of comments

[acoulton]

In our own projects we've gone with xxTestCase to follow the TestCase base class naming of phpunit, but I think AbstractXXXTestBase is also fine.

[uuf6429]

I think I like -TestCase better, I'll go for that.

[acoulton]

Thanks 😀