testing: reduce memory allocation in Helper
testing: reduce memory allocation in Helper

Store the PC instead of the string name of the function, and defer
that conversion until we need it.

Helper is still relatively expensive in CPU time (few hundred ns),
but memory allocation is now constant for a test rather than linear in
the number of times Helper is called.

benchstat:
name        old time/op    new time/op    delta
TBHelper-4    1.30µs ±27%    0.53µs ± 1%   -59.03%  (p=0.008 n=5+5)

name        old alloc/op   new alloc/op   delta
TBHelper-4      216B ± 0%        0B       -100.00%  (p=0.008 n=5+5)

name        old allocs/op  new allocs/op  delta
TBHelper-4      2.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

Change-Id: I6565feb491513815e1058637d086b0374fa94e19
GitHub-Last-Rev: c2329cf225dab6de7309af3583daa011bafb9a62
GitHub-Pull-Request: golang/go#38834
Reviewed-on: https://go-review.googlesource.com/c/go/+/231717
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Trust: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

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