Coverage and fixes for Condition.pm.

Coverage and fixes for Condition.pm.

* lib/Automake/Condition.pm (new): Catch common programming
errors better by checking type of passed argument before
munging them to all be strings through split.
* lib/Automake/tests/Condition.pl (test_basics): Also test
->human.
(test_merge): New function, test ->merge, ->merge_conds,
->strip.
* lib/Automake/tests/Condition-t.pl (test_basics, test_merge):
Likewise changes, but including state copies across thread
creation.
* lib/Automake/tests/Cond2.pl: New test for programming error.
* lib/Automake/tests/Cond3.pl: Likewise.
* lib/Automake/tests/Makefile.am (TESTS, XFAIL_TESTS): Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>