As well as:
* Remove redundant `=== null`. The `isset()` before does this already.
* Use convenient PHPUnit shortcuts.
Change-Id: Ibef571e53a48c443d7798fee8abbc2624fbad225
Clear up a bunch of phpcs ignores by documenting many methods.
Also remove Scribunto_LuaError::setLineMap(), which has apparently never
been used since it was added in Ia51f439e.
Change-Id: I763bcdbc7edbbb8e4600495a03acca3439fc0ec9