tests/.gitignore: don't ignore docker tests

The .gitignore was being a little over enthusiastic hiding files.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/.gitignore

@@ -9,6 +9,7 @@ qht-bench
 rcutorture
 test-*
 !test-*.c
+!docker/test-*
 test-qapi-commands.[ch]
 test-qapi-events.[ch]
 test-qapi-types.[ch]