file test: Remove FIXME about FIFO. It is already tested in mknod test.
file test: Remove FIXME about FIFO. It is already tested in mknod test.

PiperOrigin-RevId: 305546584

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor