oft: add --profile command line option

This produces a Python profile of the testcases. The resulting profile.out file
can be read with the pstats module, RunSnake, or pyprof2calltree.
oft: add --default-negative-timeout command line option

This option controls how long OFTest waits for an event that we don't expect to
occur. It replaces several hardcoded values.
add latency tests

These tests are mostly helpful for deriving --default-negative-timeout.
testutils: fix typo
Merge into master from pull request #121:
Profiling and configurable negative timeouts (https://github.com/floodlight/oftest/pull/121)