Exclude unit tests from Phpstan

Main reason is that Phpstan does not respect phpunit methods like:
`markTestSkipped()` and `markTestIncomplete()`.

Since tests is pseudo code/tooling I choose to take the approach to skip
Phpstan scanning in out unit tests.

Read more here:
phpstan/phpstan-phpunit#52 (comment)

phpstan.neon.dist

@@ -5,6 +5,8 @@ parameters:
     - web/modules/custom
     - web/profiles/dpl_cms
     - web/themes/custom
+  excludePaths:
+    - **/tests/src/Unit/*
   ignoreErrors:
     - '#Unsafe usage of new static\(\).#'
     # Drupal Form API makes extensive use for arrays which we cannot provide