Allow test classes to specify the config object exceptions

Given that we are only making the existing functionality configurable by moving it to base class that's why I think it's a task. This is good addition and very helpful for contrib.
Patch looks good as is but we also use static:$module pattern in TestBase and then merge all the arrays in setUp method. Do you think we should also do that here as well so that no test can overwrite the existing array. Other then that I think this change needs change record.

I think it is worth adding change record or at least update All TestBase derived tests now enforce strict configuration schema adherence by default. Patch looks good now.

+++ b/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php
@@ -44,13 +44,23 @@ class ConfigSchemaChecker implements EventSubscriberInterface {
+  protected $exceptions = array();
...
+    $this->exceptions = $exceptions;

+++ b/core/modules/simpletest/src/TestBase.php
@@ -1585,4 +1601,23 @@ public function getTempFilesDirectory() {
+      if (property_exists($class, 'configSchemaExceptions')) {

Different property names are used here.

Nevermind, configSchemaExceptions is used on the test base and exceptions on the event.

Setting back to RTBC.

I started to rename some things on commit and make everything more consistent. I got too large so uploading a new patch. I think that using the word exceptions is confusing when the ConfigSchemaChecker throws exceptions. I've changed it to be exclusions and I've made all the doc blocks consistent.

I agree this change is rc eligible.

+1 to RTBC.

Looks good. Ticking credit box for @jibran for reviews.

Pushed to 8.0.x.