gtester: Run tests only once on 'make check'

gtester: Run tests only once on 'make check'

The gtester makefile runs 'make test' on 'make check' and also recurses
on 'make test'.  automake recurses on 'make check'.

This means that every level of recursive make that we go through results
in the tests running twice as many times.  If you type 'make check' at
the toplevel, for example, tests in gio/tests/ run 4 times.

Fix that by introducing a 'test-nonrecursive' target and using that one
from 'make check'.  'make test', 'make test-report', etc. are still
recursive.