tests: fix run-tests default values in Test constructor
As far as I can tell, default values are evaluated far earlier on
Python 3.6 than 2.7, meaning we've got to be more careful about when
we read the defaults dictionary for the kwarg defaults. Sigh.
With this change, test-run-tests.t is significantly less of a mess
under 3.6.
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t