pb_tests: Add tests for the ib_srp code, update README

As we only support fast IO failing for now, we test exactly only
three things:

1. set block layer timeout through sysfs
2. hot-add a LUN with and without the timeout set
3. test fast IO failing for mainline behavior (no fast IO failing),
   for block layer timeout (15s) and QP timeout (35s)

Signed-off-by: Sebastian Riemer <sebastian.riemer@profitbricks.com>