cmd/bent: modify test to avoid rebuilding bent
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 <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/benchmarks,golang/benchmarks