Whenever a test appears in multiple suites, the rules generated
by mtest2make are currently running it twice.  Instead, after
this patch we generate a phony target for each test and we have
a generic "run-tests" target depend on all the tests that were
chosen on the command line.  Tests that appear in multiple suites
will be added to the prerequisites just once.

This has other advantages: it removes the handling of -k and
it increases parallelism.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>