The tests in doc/progs appear to have been originally written
for use with the old test driver. At some later point,
they acquired their own test driver.
Both ran tests in serial.

This CL rewrites the current test driver in Go,
runs tests concurrently, and cleans up
historical artifacts from the old drivers.

The primary motivation is to speed up all.bash.
On my laptop, using tip, this CL reduces doc/progs test
wall time from 26s to 7s. The savings will remain
even when the compiler gets faster. Using Go 1.4,
this CL reduces test wall time from 15s to 4s.

Change-Id: Iae945a8490222beee76e8a2118a0d7956092f543
Reviewed-on: https://go-review.googlesource.com/8410Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>