Remove MediaWikiTestCase dependency

- Any change to the `MediaWikiTestCase` can potentially disrupt test scenarios.
- Introduce an independent `MwUnitTestDatabaseInstaller` that replicates the `MediaWikiTestCase` DB cloning procedure.
- `MwDBaseUnitTestCase` itself only relies on `PHPUnit_Framework_TestCase`
Remove MediaWikiTestCase from MwIntegrationTestCase
Remove MediaWikiTestCase from remaining tests
Merge pull request #311 from SemanticMediaWiki/less-mw-testcase

Remove MediaWikiTestCase dependency