rtimer-tests/: add README; trim "one-timer" test; clean up sim.c a little
rtimer-tests/sim.c (reset_sim): reset also the "new" variables
rtimer-tests/: use file name as test case prefix

... and don't include it in the descriptions.
rtimer-tests/sim.c: make invocation count zero-based; iterate over all invocations

This eliminates the "do nothing" calls with count = 0. We still get the
same effect simply by having the probe in code the simulation doesn't
reach.

Furthermore, the simulator did not test all iterations but only the
first one.
rtimer-tests/README: describe comments; explain the issue found