Make the tests runnable on a read-only src. To do this you must make sure

Make the tests runnable on a read-only src. To do this you must make sure
that you create one of the object directories make knows (see make(1)).
This uses the -C flag, so add a test that checks that make actually accepts
-C. Also fix the test that selects csh via the .SHELL target to work for
tcsh users too.

This commit renames shell_test to shell_test.sh. There is no history
to preserve so go without a repo-copy.

Reviewed by:	ru