Null test.
Stop without starting fails with "Gc.Memprof.stop: no profile running."
Start without stopping fails with "Gc.Memprof.start: already started."
Discard without stopping fails with "Gc.Memprof.discard: profile not stopped."
Second discard fails with "Gc.Memprof.discard: profile already discarded."
Double profile.
Double profile with single discard.
Double profile, discarding both.
Double profile, discarding both at end.
Double profile, discarding in reverse order.
Discarding old profile while sampling.