#12 portability: test with ash and base shells

[Julie Fleischer]

If someone is interested in the portability of the POSIX
test suite (esp. if it is POSIX compliant on all OSs), it
would be useful to test against the ash shell and the
base shell.

See mailing list discussions regarding the POSIX
compliance of certain POSIX expressions (esp. [[ and
((). Testing on these other shells helps expose these
types of issues.

Anyone interested in the portability of the POSIX test
suite can work on this feature request.