test/ofproto: Improve test 'controller action without megaflows'

test/ofproto: Improve test 'controller action without megaflows'

Commit af7535e7dbeb9 expanded the test to check the output
of meter stats, but without strip out the duration time.
This makes the test sensitive to the speed of
the machine that runs the test. Strip away the timing information
to improve test reliability

Fixes: af7535e7dbeb9 (ofproto: Meter slowpath action when action
                      upcall meters are configured)
Signed-off-by: Andy Zhou <azhou@ovn.org>