[merge] add TestNotApplicable and test documentation

[Ian Clatworthy]

Martin Pool wrote:
> I want to add some per-inventory-implementation tests.  I've added
> some documentation of the per-implementation test framework, and also
> a distinct exception for 'test not applicable'.

bb: tweak

Comments below.

> +    def test_not_applicable_demo(self):
> +        # just so you can see it in the test output
> +        raise TestNotApplicable('this test is just a demonstation')
> +

Spelling error in demonstration.

> === modified file 'doc/developers/HACKING.txt'

> +======================= ======= ======= ========
> +result                  strict  default lax
> +======================= ======= ======= ========
> +TestSkipped             pass    pass    pass
> +TestNotApplicable       pass    pass    pass
> +TestPlatformLimit       pass    pass    pass
> +TestDependencyMissing   fail    pass    pass
> +KnownFailure            fail    fail    pass
> +======================= ======= ======= ========

As Aaron has pointed out in an earlier email, it would be good to be
able to add tests known to trigger bugs and to record those using
KnownFailure. I think 'pass pass pass' for KnownFailure is therefore a
better choice here.

Ian C.