timeval: Add "time/stop" unixctl command, for use in unit tests.

timeval: Add "time/stop" unixctl command, for use in unit tests.

Although we try to avoid it, some unit tests are necessarily
timing-sensitive.  The new "time/stop" command that this commit adds should
help with that, by preventing time from advancing from the viewpoint of
the OVS "timeval" functions except when "time/warp" explicitly advances
the current time.  This should allow the unit tests that need it to become
reproducible regardless of the speed at which the tests run.

This commit adds one use of "time/stop" to the unit test suite, in the one
timing-sensitive test of which I am currently aware.

Bug #9782.
Reported-by: Tim Chen <tchen@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>