cmd/bench: change goTestSubrepo to run sub repo experiment as toolchain experiment.

Change-Id: I3ccfbb22422034c01814fc4a247518c4d28e2cda
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/458917
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dylan Le <dungtuanle@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
2 files changed