testing: support pausing cpu profiling in benchmarks

[masiulaniec]

The benchmarking part of the "testing" package supports stopping timers while
performing complex initialisation. Perhaps something analogous could be added for cpu
profiling, so that -cpuprofile can produce a data file that excludes samples for the
complex initialisation.

The current workaround is to avoid -cpuprofile in favour of calling
runtime.StartCPUProfile from within the benchmark.

[griesemer]

Comment 1:

Labels changed: added repo-main.

[rsc]

This is infeasible.