cmd/bent: add some more benchmarks

listed in benchmarks-all.toml, not yet in the default
"benchmarks-50" set.

Change-Id: If4378bbf5f2574b1987389199160c27bfdf0f302
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/362374
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>