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
