tests/tpm: fix tpm_util_swtpm_has_tpm2()

tests/tpm: fix tpm_util_swtpm_has_tpm2()

Using g_spawn_async_with_pipes() is more complicated than running the
sync version. The async version returns a file descriptor for stdout, which may
not be fully read. Sometime "--tpm2" will failed to be read, and will
cause the related test to be silently skipped.

Use g_spawn_sync() instead, simplifying the code and fixing the race.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>