gpu: nvgpu: unit: fifo: fifo unit test

This unit test covers most of the nvgpu.common.fifo.fifo module lines
and almost all branches.

Jira NVGPU-3697

Change-Id: I5722277a3e1630a902f63b707eb3de1c4e1876b0
Signed-off-by: Vedashree Vidwans <vvidwans@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2237796
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

userspace/SWUTS.h

@@ -47,6 +47,7 @@
  *   - @ref SWUTS-fifo-engine-gp10b
  *   - @ref SWUTS-fifo-engine-gv100
  *   - @ref SWUTS-fifo-engine-gv11b
+ *   - @ref SWUTS-fifo-fifo
  *   - @ref SWUTS-fifo-fifo-gk20a
  *   - @ref SWUTS-fifo-fifo-gv11b
  *   - @ref SWUTS-fifo-pbdma