Allow specifying '--build' and '--run' flags for run_net_test.sh.

They don't really do anything (as they're the default),
but it's less effort to edit --nobuild to --build
and back again while dev/testing.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I6cebde6e7ff7559c0195c487178e3ae4a818ecaa