Improve test coverage in includes/password

Improve test coverage in includes/password

From 21% to 82%.

* Added missing @covers, broadened @covers where appropriate.
* Added tests for some code that lacked them.
* Added a parameter to control the use of hash_pbkdf2() so that the pure
  PHP fallback could be tested. In the non-fallback test, force the use
  of the extension, and mark it skipped if it is not installed.

Bug: T167003
Change-Id: I987e1a89ec343907f4ead7f6192b2d4deb58ac16