proposal: testing: reuse the iteration number of the first round when count > 1

[shawndx]

We often run all benchmarks of std packages with -count=5 or higher, it usually takes 5-6 hours to finish on our x86 and arm64 servers, the overall turnaround time might be reduced a lot if we can skip the auto adjustment from round #2 and reuse the iteration number computed before.

With an experimental change I got the following numbers:

                                    old                                 new
package math/big            2865.780s                              2242.640s
package runtime              5936.460s                              4926.073s
package encodng/json     486.691s                                321.370s

I realized same attempt was made before but then dropped due to benchmark regressions reported in #25622, a change to computing the 'grain' value of RunParallel proposed in #37996 shall avoid the regression. Furthermore this feature could be guarded by an option if necessary.

[seankhliao]

I believe this is now possible with https://pkg.go.dev/testing@master#B.Loop scheduled for go1.24.