ipcscale/sem-waitforzero: Add multiple threads/core

Add the ability to put multiple threads on the same core.
The new feature itself is not really useful, it's just
the first step for merging sem-waitforzero and sem-pingpong:

For a ping-pong test, it makes sense to either intentionally
put both partners on the same core (threadspercore=2) or to
put them on different cores (threadspercore=1).

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
ipc/sem-waitforzero: rename to sem-scalebench

Rename sem-waitforzero to sem-scalebench:
sem-pingpong will be merged into the same test.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
sem-pingpong: remove test

The test is merged into sem-scalebench.
Advantage:
- Ability to use interleaves
- less copy&paste

Disadvantes:
- less pretty output
sem-scalebench: Merged app for waitforzero and ping-pong

New, unified app for both tests