Replace random delay test case with a competing flow case in QuartcBidiTest.

This test case runs for 15 seconds normally, then introduces a competing flow
that transfers a burst of 50 kb of data in each direction.

Per-packet random delay isn't very realistic.  A burst of competing traffic is a
much more realistic scenario for testing how well the Quartc transport responds
to an unpredictable change in delay.  For example, the user might make an HTTP
request and load a web page while streaming media.

A couple of ideas for future competing flow tests include persistent competition
or multiple small bursts that occur throughout the test.

gfe-relnote: n/a (test only)
PiperOrigin-RevId: 246586456
Change-Id: I50d090135e2193c17c7249904a4a996be43f3f00