qemu-iotests: flush after every test

This makes it possible to see what is happening, even if the output of
"make check-block" is not sent to a tty (for example if it is sent to
grep or tee).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qemu-iotests/testrunner.py

@@ -341,6 +341,7 @@ class TestRunner(ContextManager['TestRunner']):
             elif res.status == 'not run':
                 notrun.append(name)
 
+            sys.stdout.flush()
             if res.interrupted:
                 break