tests: Rename and abstractify custom TestCase base classes (#41935)

PHPUnit 10+ requires that test class names match the filename, PSR-4
style. We've written a PHPUnit sniff to help find mismatches, but to
work properly it needs any custom TestCase base classes to be abstract
and have names ending in "TestCase" or "TestBase".

This PR adds `abstract` where necessary and renames classes where
necessary.

In a few places this has already caused other existing PHPUnit rules to
start fixing things. 😀

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/13500864408

Upstream-Ref: Automattic/jetpack@18072cc

vendor/composer/installed.json

@@ -7,7 +7,7 @@
             "dist": {
                 "type": "path",
                 "url": "/tmp/jetpack-build/Automattic/jetpack-device-detection",
-                "reference": "da41451c1401be76130cea3e326bef27b8ae9ceb"
+                "reference": "e3566754d27e5a0421fe1d9a75687b917ce3ae31"
             },
             "require": {
                 "php": ">=7.2"

vendor/composer/installed.php

@@ -13,7 +13,7 @@
         'automattic/jetpack-device-detection' => array(
             'pretty_version' => '3.0.0',
             'version' => '3.0.0.0',
-            'reference' => 'da41451c1401be76130cea3e326bef27b8ae9ceb',
+            'reference' => 'e3566754d27e5a0421fe1d9a75687b917ce3ae31',
             'type' => 'jetpack-library',
             'install_path' => __DIR__ . '/../automattic/jetpack-device-detection',
             'aliases' => array(),