qemu-ga: do not run qga test when guest agent disabled

When configure with --disable-guest-agent, make check will fail with:
ERROR:tests/test-qga.c:74:fixture_setup: assertion failed (error == NULL):
 Failed to execute child process "/home/xx/qemu/qemu-ga" (No such file or
directory) (g-exec-error-quark, 8)
make: *** [check-tests/test-qga] Error 1

This check was commented out by bab47d9a75a. I think that was by
mistake, because the commit message of that commit didn't mention
this change.

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
