Avoid syntax error in ash.

Avoid syntax error in ash.

* lib/autotest/general.m4 (AT_INIT) <driver loop>: Avoid syntax
errors on shells that don't recognize <>.
* tests/autotest.at (AT_SKIP_PARALLEL_TESTS): Also skip parallel
tests for this reason.   Skip based on the shell to be tested,
not the shell driving the testsuite.
(parallel syntax error): Rearrange similar to previous patch.
(parallel test execution): Defer skip until after serial tests.

Signed-off-by: Eric Blake <ebb9@byu.net>