Code review comment for lp:~lifeless/bzr/test-speed

This finishes the thread I started tugging on this morning.

It does two basic things:
 - moves tests that are not for the bzr selftest command line out of
   bzrlib.tests.blackbox.test_selftest
 - where possiblestops invoking child processes to test how we call such
   processes, instead extending the existing idioms for intercepting
   such calls to test how the calls are setup.

This saves another 50% of test time in 'test_selftest' tests over my
patch earlier today.

-Rob

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