BUG: runtests: chdir away from root when running tests

This makes it possible for subprocesses possibly launched by the test to
import the module under testing.

Also, do not pass --debug to setup.py build, since this can make the
build require Python debug symbols to be installed (on windows).
Merge pull request #2986 from pv/runtests-fix

BUG: runtests: chdir away from root when running tests