cmd/bent: extend "copy subdirs for running tests" to regular files

This is necessary to run e.g. spexs2.

Generalized the field name, also fixed the lack of logging some
steps for -v.

Change-Id: Ie898a86389a8de8884dd0fe6c67fc995aa18216b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/379536
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>