vala tests: avoid spurious failure with older GObject

vala tests: avoid spurious failure with older GObject

* t/vala2.sh: In the 'PKG_CHECK_MODULES' call in 'configure.ac',
don't require gobject >= 2.10, but just >= 2.4: that is enough in
order for the test to pass.
* t/vala3.sh: Likewise.
* t/vala5.sh: Likewise.  Also, skip the test instead of failing
if the ./configure invocation fails: that is likely due to the
fact that the GObject library is too old or missing, and that is
not automake's fault.

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