Initialise flag global in test to avoid side effects.
Initialise flag global in test to avoid side effects.

The flag value isn't initialised because we never call flag.Parse, and
testutil.TestSetFlag doesn't seem to reset the value as a result -- probably
the value is nil?

Anyway, manually setting the value at the start of the test seems like a nice
hack.  Really we should avoid the flag value here and pass it in to the
pushOptions somehow.

SuperQ/mtail,google/mtail,google/mtail,SuperQ/mtail