proposal: testing: add TB.SetGOMAXPROCS

I propose adding a SetGOMAXPROCS. This idea is inspired by TB.Setenv.

As seen Go source codes, too, if it sets GOMAXPROCS and it unsets this value after cleanup, we must write the following ways.

defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(n))

prev := runtime.GOMAXPROCS(n)
defer runtime.GOMAXPROCS(prev)

The writing style is general, if you know Go specification. But I think we can provide better way to achieve.

I submitted a CL. If this CL isn't been necessary, I don't mind to close it. Thanks.

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