cmd/bent: refactor benchmark running

pull the runner out into a single function.
fix a bug in the PGO feature

Added toml benchmark.

Change-Id: I3be36baddb5a7ca258a1a6872351af5146a1d641
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/565115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>