Simple running of just one test case

With this you can run a single test case only with

  make check T=/testcase/name

This does not currently cover the Python tests.
Makefile.am: Fix handing of default vala flags

$(AM_VALAFLAGS) is not automatically being used apparently. Drop the
unnecessary --enable-experimental and add a minimal glib requirement to >= 2.32
instead, so that we can use threads properly.