vala tests: few adjustments and improvements

vala tests: few adjustments and improvements

* t/vala4.sh: Don't run 'libtoolize', it's not truly required.
($required): Don't require 'libtoolize'.
(configure.ac): Don't invoke 'AC_PROG_LIBTOOL'.
* t/vala-mix.sh ($required): Require 'cc'.
Avoid some calls to 'framework_failure_', they were only useful when
this test was xfailing (to ensure it wasn't failing for the wrong
reasons).
Ensure the stamp file is created in the source directory.

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