DiffPluginTest contains 14 test methods, split into multiple classes

We could also group the test methods as:
function testPlugins() {
doTestCommentPlugin();
doTestCorePlugin();
...
}

This removes most time = setup. And the tests are indeed pretty isolated and don't need a new environment.

BTW each method should contain a @see reference to its corresponding plugin class.

Prio down, although still happy to see this cleanup happen.

Added method and @see references to the plugins.

Moving the failing one into a separate test again.

Committing this temporary improvement.

Strange: I have lots of failing tests locally. Didn't figure out yet why.
Back to needs work to to apply the pattern to the other ones that failed.

The test will fail locally if you have the Visual Inline libraries installed.

Created issue to clean-up / fix test fails related to htmldiff: #2823903: Make tests pass when html diff library is present

Keeping this issue needs-work to further split the tests / reduce test methods in DiffPluginTest.

1. Added new DiffEntityPluginTest with 3 methods extracted from DiffPluginTest
-testEntityReferencePlugin
-testFilePlugin
-testImagePlugin
2. Configured DiffPluginTest and removed default configs that weren't needed
Tests passed locally

Small fix in the DiffPluginTest, added description and deleted extra line.

+++ b/src/Tests/DiffEntityPluginTest.php
@@ -0,0 +1,268 @@
+class DiffEntityPluginTest extends DiffTestBase {
...
+  public function testEntityReferencePlugin() {
...
+  public function testFilePlugin() {
...
+  public function testImagePlugin() {

That's now 3 cases in newly created class.
@Berdir, did you intend this or a test class per plugin, or stay with the current approach and convert them to a doTestXYZ pattern because they don't necessarily need a full reinstall - except that they currently accidentally clash?

Shifted some stuff around. The PluginTest still needs division. But all other tests now use variables for all ID's and made the views configuration optional. Their runtime is much lower.

And now with all the things separated.
Thought i would do more test classes - one per plugin - but that was not needed as the plugins can be tested with doXYZ approach easily.
Test run time down nicely. Just below 1m30 for me.

Retryyyy..

Oh... Committed with declaring the base class abstract. Yippie! :-)