init.sh: fix EXEEXT shims to work also for names like test-prog

init.sh: fix EXEEXT shims to work also for names like test-prog

* tests/init.sh: Re-exec a better shell, when needed.
If the current shell lacks support for posix $(...), an init.sh-using
test will now try to find a shell that supports that.  If EXEEXT is
nonempty, we also require support for hyphen-in-alias-name and shell
substitutions like ${var#glob}.  Failure to find such a shell results
in a skipped test.