tests: improve performance of device-introspect-test

tests: improve performance of device-introspect-test

Total execution time with "-m slow" and x86_64 QEMU, drops from 3
minutes 15 seconds, down to 54 seconds.

Individual tests drop from 17-20 seconds, down to 3-4 seconds.

The cost of this change is that any QOM bugs resulting in the test
failure will not be directly associated with the device that caused
the failure. The test case is not frequently identifying such bugs
though, and the cause is likely easily visible in the patch series
that causes the failure. So overall the shorter running time is
considered the more important factor.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
[thuth: Add the tree check to test_device_intro_none() and
 test_device_intro_abstract(), too, just to be sure...]
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200710060719.22386-1-thuth@redhat.com>