Copy all <srcdir>/python to <build-dir>/python before running bin/testharness.

This means all the necessary python is available to both testharness itself and
the tests (testharness will add <build-dir>/python/ to PYTHONPATH when run as
<build-dir>/bin/testharness). Using that directory to import python modules
from also means <srcdir>/python will no longer be clobbered with .pyc files
when running make test.
Someone thought it was a good idea to also put makefiles in the src/ directory.

More stripping of relative paths to bin/