tests: explicitly wait for the PG to finish before looking for capture file

tests: explicitly wait for the PG to finish before looking for capture file

Rather than only using time-based method of periodically checking
whether the pcap file appeared, first check that the packet generator
has stopped. To make this change fail-safe, have a 5-minute timeout
on this activity, just in case the things go terribly wrong.

Type: test
Signed-off-by: Andrew Yourtchenko <[email protected]>
Change-Id: Id16b2802b2de8a4cafb5d9f0a8c9ba62ec89dc32
(cherry picked from commit 3d36f19a0febaed532bd255a150504f7af8f18c2)