tests: disable Ming test if Ming unavailable

And remove Ming from test-requires, as requested by infra/release team.

Fixes: bug#1158405

Change-Id: I83023ca865b54c5c94bf118122914c7e89e007c8
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "tests: disable Ming test if Ming unavailable"