cmd/bent: modify test to avoid rebuilding bent

this allows the test to run on more platforms, and also to
run more quickly.

Fixes golang/go#49964.

Change-Id: I8b0661af6f00d40e1580652a6594312544b3e0df
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/369159
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
1 file changed