Tone down note about not pinning threads
The note mdrun prints to stderr and log file when not pinning
threads is now a short note instead of the old failure message,
since not pinning is often a choice, not a failure. With more than
one thread, a failure or longer explanatory note will anyhow have
been issued before.
Also removed a warning about OpenMP set affinity when the total
number of threads was chosen by mdrun.
Updated the thread affinity tests for the new message. Added tests
that are now possible with the recently introduced auto thread count
variable.
Fixes #2088 and #2319
Change-Id: I1411a1ce6e222d22da8d70bf7bab2c9bb7564507