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 bab47d9a75. 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

tests/Makefile

@@ -83,7 +83,9 @@ check-unit-y += tests/test-crypto-cipher$(EXESUF)
 check-unit-y += tests/test-crypto-secret$(EXESUF)
 check-unit-$(CONFIG_GNUTLS) += tests/test-crypto-tlscredsx509$(EXESUF)
 check-unit-$(CONFIG_GNUTLS) += tests/test-crypto-tlssession$(EXESUF)
-#check-unit-$(CONFIG_LINUX) += tests/test-qga$(EXESUF)
+ifneq (,$(findstring qemu-ga,$(TOOLS)))
+check-unit-$(CONFIG_LINUX) += tests/test-qga$(EXESUF)
+endif
 check-unit-y += tests/test-timed-average$(EXESUF)
 check-unit-y += tests/test-io-task$(EXESUF)
 check-unit-y += tests/test-io-channel-socket$(EXESUF)