slices: add sort benchmark for sorted strings

slices: add sort benchmark for sorted strings

For #60777

Change-Id: I424535ce6454156c61af2f299228630ee304d165
Reviewed-on: https://go-review.googlesource.com/c/go/+/503815
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Eli Bendersky <eliben@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>