tests: convert the 'file://\$TESTTMP' rule to an automatic substitution
The rule only triggered on non Windows platforms, even though Windows also
required an adjustment. Automatic seems better.
The aggressive globbing in test-subrepo-svn.t was found and rewritten by the
substitution.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.