autobuild: harden samba-xc test suite

autobuild: harden samba-xc test suite

Add more checks which directly test the behavior of
--cross-answers and --cross-execute.

Previous test tested things in a round-about way, checking
that running in all three modes (native, cross-execute,
cross-answers) yields the same result. It was vulnerable
to a degradation in which cross-compilation modes didn't
work at all and were running native tests, which is
what happened with the upgrade of waf.

The added tests check the following:
- That cross-excute with cross-answers sets the cross-answers file
- That the content of cross-answers file actually affects the build
  configuration
- That a missing line in cross-answers fails the build

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13846

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Sun Oct 20 13:29:58 UTC 2019 on sn-devel-184