Patch series on run-tests.py reusability

[Gregory Szorc]

It looks like the list swallowed my initial email describing the 
run-tests patch series I just submitted. Doh.

The goal of this series of patches is to make run-tests.py reusable from
contexts beyond ./run-tests.py.

While this series stops far short of the ultimate goal of making
run-tests.py an importable module and thus reusable as a library with
proper handling of global state (read: no global state), this series
does enable run-tests.py to be imported and for main() to be called from
other entry points.

This approach is successfully employed at
https://hg.mozilla.org/hgcustom/version-control-tools/file/7085790ff3af/run-mercurial-tests.py