Reduce benchtests time

Reduce the total time taken by benchtests.  The malloc thread test takes 4
minutes to run which is significantly more than most other tests. Reduce
this to a more reasonable 40 seconds.  The math tests take 10 seconds each,
eventhough all they do is loop on the same input.  Anything more than 1
second runtime is way overkill, so set the limit to 1 second.

	* benchtests/Makefile (BENCH_DURATION): Set to 1 second.
	* benchtests/bench-malloc-thread.c (BENCH_DURATION): Set to 10 seconds.