tests: make pg_start() wait until pg completes

tests: make pg_start() wait until pg completes

A sizable number of tests call pg_start() to get the packets flowing and then
immediately expect to have the entirety of the packets gone through.
This works on powerful and unstressed hardware, but fails in beautifully random
ways under load.

This also necessitates the complicated logic of remembering the "zombie captures",
then sleeping for some time before cleaning them up....

The solution is simple: in pg_start(), start the generators, wait till they
all finish, clean up, done.

Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: I930e51b7aae39c9841d22dd905a4d13a465a672b
Type: test