runtime: convert flaky semaphore linearity test into benchmark

runtime: convert flaky semaphore linearity test into benchmark

Also, add a benchmark for another case that was originally tested.

Also also, remove all the dead code this now creates.

Fixes #53428.

Change-Id: Idbba88d3d31d38a8854fd5ed99001e394da27300
Reviewed-on: https://go-review.googlesource.com/c/go/+/412878
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>