cmd/dist: run various one-off tests in parallel

Takes 15% off my all.bash run time
(after this and earlier CLs, now down to 3½ from 5½ minutes).

For #10571.

Change-Id: Iac316ffb730c9ff0a0faa7cc3b82ed4f7e6d4361
Reviewed-on: https://go-review.googlesource.com/18088Reviewed-by: Ian Lance Taylor <iant@golang.org>