[merge] add TestNotApplicable and test documentation

[Martin Pool]

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'.  This is meant to
handle cases where a per-implementation test is not applicable to one
of the implementations because it indicates it does not support that
feature.  We have previously been either just returning from the test,
or raising TestSkipped.  I don't like the former because it looks like
the feature was tested but it wasn't; Robert doesn't like the second
because (iirc) it is conflated with other meanings for TestSkipped.

-- 
Martin
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