tests: check for file existance in a portable way
The precise output of ls is highly OS specific, so just use the simpler
test command.
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/.."