Rename jhelp.pl to thelp.pl and make it a generic test helper.

Rename jhelp.pl to thelp.pl and make it a generic test helper.

* tests/thelp.pl: Rename from tests/jhelp.pl.
(op): Use names instead of options for the operations.
(op): Add new operations for sleep, mkdir, and rm.
(op): Enhance wait to time out
* tests/run_make_tests.pl: Add a new #HELPER# replacement
(subst_make_string): Use fully-qualified path to thelp.pl
* tests/scripts/features/parallelism: Update to use thelp.pl
and the new named operations.  Use thelp.pl sleep instead of
system-specific sleep commands.
* tests/scripts/features/output-sync: Update to use thelp.pl
instead of complex shell scripts.
* Makefile.am: Distribute tests/thelp.pl instead of tests/jhelp.pl