runner: Take the number of workers as a flag.

Default to the number of CPUs. Avoids the tests launching 64 valgrinds in
parallel on machines without gobs of memory.

Change-Id: I9eeb365b48aa7407e303d161f90ce69a591a884c
Reviewed-on: https://boringssl-review.googlesource.com/1375
Reviewed-by: Adam Langley <agl@google.com>