run-tests: also catch double-escapes for $TESTTMP
On Windows the `$TESTTMP` string "repr" is different than its value, because `\`
become `\\`. We introduce a new `$STR_REPR_TESTTMP` substitution to match that
too.
Differential Revision: https://phab.mercurial-scm.org/D11001
(binary:application/octet-stream)