cmd/bent: always copy testdata and other specified dirs into run directory

Some benchmarks expect to scribble in testdata, some benchmarks expect
other directories to exist.  Allow the others to be specified, always
copy directories into testdata.

Change-Id: I3f3d47d95610101362bf645289c7477ee05010c2
Trust: David Chase <>
Run-TryBot: David Chase <>
TryBot-Result: Gopher Robot <>
Reviewed-by: Michael Pratt <>
2 files changed