cmd/go: split out cmd/go/internal/test

This is one CL in a long sequence of changes to break up the
go command from one package into a plausible group of packages.

This sequence is concerned only with moving code, not changing
or cleaning up code. There will still be more cleanup after this sequence.

The entire sequence will be submitted together: it is not a goal
for the tree to build at every step.

For #18653.

Change-Id: I2d0ccdb84814537ab8b8842aa1b5f5bc0a88a0fc
Reviewed-on: https://go-review.googlesource.com/36198


Reviewed-by: David Crawshaw <crawshaw@golang.org>