tests: do not change sys.path, it can break loading cext.parsers...
tests: do not change sys.path, it can break loading cext.parsers
When running this tests with run-tests, the prefix would resolve
mercurial.cext to the source tree and the attempt to load
mercurial.cext.parsers would therefore fail since it doesn't exist in
it. With the regular search path from run-tests, it is picked up from
the temporary prefix correctly.
Differential Revision:
https://phab.mercurial-scm.org/D4910
