Add option to read lists of tests from file.

[rsmmr]

-i FILE, --tests-file=FILE
    Loads the list of tests to execute from a file. Each line in the
    file is interpreted as the name of a test, or a group of tests, to
    execute, just like the tests would be specified on the command
    line. Empty lines and lines starting with ``#`` are ignored. (This
    format is compatible with that of the ``btest`` `StateFile
    <state_file_>`_.)

[bbannier]

I think my points from #55 which we probably should link still stand, but I am not strongly opposed to this. Maybe but for the options conflict checking this is simple enough, though btest $(cat <FILE>) might have less limitations (e.g., can both "run from file" and specify additional tests on the command line).

[rsmmr]

I think my points from #55

Oh, I totally forgot about that one and the discussion there --- just found myself wanting such an option once more. That said, I do prefer this over $(cat <FILE>): easier to both type and remember. And this isn't tied to the state file, so that should avoid any confusion about updating.