benchmark

Don't limit benchmarks with manual timers to 5x the elapsed real time.

When using CPU time to determine the correct number of iterations the
library additionally checks if the benchmark has consumed 5x the minimum
required time according to the wall clock. This prevents benchmarks
with low CPU usage from running for much longer than actually intended.

However when a benchmark uses a manual timer this heuristic isn't helpful
and likely isn't correct since we don't know what the manual timer actually
measures.

This patch removes the above restriction when a benchmark specifies a manual
timer.