tests: disable global props test for old glib

follow-up patch moves global property tests to subprocesses.
Unfortunately with old glib this causes:

tests/test-qdev-global-props.c: In function
‘test_static_prop’:
tests/test-qdev-global-props.c:80:5: error: implicit
declaration of function ‘g_test_trap_subprocess’
[-Werror=implicit-function-declaration]
tests/test-qdev-global-props.c:80:5: error: nested extern
declaration of ‘g_test_trap_subprocess’ [-Werror=nested-externs]

This function was only added in glib 2.38, and our
minimum version is 2.12.

To fix, disable the test for glib < 2.38.

Apply before that patch to avoid breaking bisect.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>