vala tests: skip in a cross compiler setup

vala tests: skip in a cross compiler setup

The "vala + pkg-config" doesn't interact well with cross-compilation:
<https://mail.gnome.org/archives/vala-list/2012-September/msg00094.html>

That is causing annoying failures in our testsuite when it's run in
cross-compilation mode.  It's not worth trying too be overly smart and
work around these problems; just skip the affected tests instead.

* t/ax/am-test-lib.sh (require_tool): A new requirement 'valac', telling
that the test needs to compile Vala-generated C files.  In particular,
this causes the test to be skipped when using a cross-compiler.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>