Renamed constants in unit tests for paced sender.
Renamed constants in unit tests for paced sender.

Since paced sender no longer has an internal multiplier, the constants
used for the unit tests were changed from supplying kTargetBitrateBps *
kPaceMultiplier at each usage to simply using the new constant
kPacingRateBps, simplifying the test code.

The function PacketsSentPerInterval was introduced as the value was
computed and explained several times over in the different tests.

Bug: None
Change-Id: Ib1cf9b40194272b1529abb02d49cae6b8732d1e6
Reviewed-on: https://webrtc-review.googlesource.com/50443
Reviewed-by: Stefan Holmer <eb0877843acc39f8ef6f7269937dee931c372d23@webrtc.org>
Commit-Queue: Sebastian Jansson <c24d1bc144eb7c11e553f59153fa17dba85ca83e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21999}
TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc