`Program.Benchmark` should take a `testing.B`

[ti-mo]

Let's go ahead with converting Program.Benchmark() to take a testing.B instead, since it feels pointless to break API twice if we're planning on doing that anyway. When we make this conversion, we can hide Reset() from the caller and report our own metric, e.g. retries to both make sure retries don't go unnoticed and don't need to be handled explicitly by the user.

Then the retries argument also becomes unnecessary as we can take b.N as the value to RunOptions.Repeat.

Originally posted by @ti-mo in #1894 (comment)