tests: Add a RX functionality test

tests: Add a RX functionality test

Use the rxperf performance testing tools to add a couple of simple
RX tests. The first moves 1Mbyte of data backwards and forwards 30
times. The second starts 30 threads, which each move 1MByte of data
once.

This is by no means an exhaustive test of RX, but the single and
multi-threaded invocations should provide a useful smoke test if
things get very broken.

Change-Id: I11267be067cf6c05a20aeb90a18ed4031502a1b1
Reviewed-on: http://gerrit.openafs.org/7244
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>