tests: use autotest as_unset rather than rerolling $unset locally.
On at least Unixware 7.1.4, the shell variable 'unset' gets set
to 'no' during the testsuite run, breaking several test cases.
We should have been using Autotest $as_unset anyway.
* tests/testsuite.at (unset): Remove test and variable setting.
(LT_AT_MAKE): Use $as_unset as provided by autotest boilerplate
instead of our own $unset.
* tests/demo.at (uninstalled libraries ave priority): Likewise.
Reported by Tim Rice.
Signed-off-by: Gary V. Vaughan <gary@gnu.org>