test-compat: create a compatibility branch for Mercurial 5.3
Mercurial 5.4 have been released, so the stable branch is no longer about 5.3.
Enable obsolescence to avoid the warning issue when obsmarkers are found
$ cat << EOF >> $HGRCPATH
> [experimental]
> evolution = all
> EOF
$ $TESTDIR/../contrib/check-compat-strings.sh "$TESTDIR/../hgext3rd/" "$RUNTESTDIR/.."