Makefile: allow setting HGTESTFLAGS in shell environment for TESTFLAGS
I keep wanting to run 'make tests', but I forget to set TESTFLAGS='-j
16' or whatever is reasonable for my machine. This lets me just set it
once in my shell settings and forget it.
Makefile: introduce testpy-% target for testing with a specifc Python
This makes it easy to do 'make testpy-2.4.6 TESTFLAGS="-j 16"' and the
Makefile will build Python if needed and then run tests (with -j 16)
with the resulting Python.
You can set the environment variable HGPYTHONS to a nice location on
your machine to cache the Python builds globally. If that's not set,
it builds them inside build/pythons.