Parametrization being a bit too broad

[Martin Pool]

I agree it's a bit overbroad.

I'd like to see some of the repository and other component classes to
turn into more of a composition of different aspects (such as knit
storage etc) and then we could perhaps express that the tests ought to
be run per different relevant implementation.

If we split them into a per_* for each different aspect I wonder if it
will get hard for people to find a test or to work out where they
ought to add tests.  On the other hand it is useful to have the tests
be structurally clear about how they're parameterized.

We'd want to be careful not to fail to test cases where it's the way
the things are combined that matters.

I think it's worth pushing on.

Martin