Rename RulesDrupalTestBase -> RulesKernelTestBase

The old name was derived from the Simpletest DrupalWebTestCase, but things have evolved and that class no longer exists. Instead, now all the tests are PHPUnit tests.

Our test class hierarchy currently looks like this:
Functional tests are subclasses of RulesBrowserTestBase which extends BrowserTestBase
Kernel tests are subclasses of RulesDrupalTestBase which extends KernelTestBase
Unit tests are subclasses of RulesUnitTestBase which extends UnitTestCase

For naming consistency and to make things easier to understand, this patch changes RulesDrupalTestBase to be RulesKernelTestBase

This change MIGHT affect contributed modules which integrate with Rules and have test cases for their Rules integration. However, I have checked the modules I know of in D8 that have put effort into supporting Rules, and none of them are subclassing RulesDrupalTestBase, so I believe this change won't affect many (if any) people. Regardless, Rules is still in alpha, so this is when we can/have to make any changes like this.