rdbms: Refactor DatabaseTest

rdbms: Refactor DatabaseTest

* Move DatabaseTest and DatabaseSQLTest to libs,
  and remove MediaWikiTestCase dependency.

* Refactor DatabaseTest to be a test of the Database abstract class,
  not of whatever current DB backend is configured by LocalSettings.

  - Remove most switches/conditionals and other tests for specific
    database backends. Move those to individual test classes for
    those backends instead.
  - Some tests appear to have been integration tests for the PHP driver
    and/or the db backend itself. Moved to a new DatabaseIntegrationTest.
  - Now that only the abstract Database is invoked, the test runs a bit
    faster (no real connections/queries).

* Add missing @covers tags, and remove or fix broken ones
  (follows-up 26e52f0c491a4).

Change-Id: I9dc4a558e701d00e95789e7eb8e02926783b65ad