Cmd tests timeout and splitting tests

[Dan Kegel]

Hi Octavian,
the intention was to just add new TESTCMD resources into rsrc.rc
when test_builtins.cmd got too big.... but I never thought
the tests would be so slow, splitting them by adding new rsrc.rc
entries wouldn't help a timeout problem.

I'd go for solution #3, too, but it's troubling that test names
would be duplicated in the Makefile.in and in rsrc.rc.

An awful thought just occurred to me: we could make test_builtins.cmd
itself take a parameter saying which subtest to run.
That would avoid needing to modify rsrc.rc,
but you'd still need to figure out how to iterate through the tests.
- Dan