cmd/go: show FAIL for errors during test setup

For example, if an x_test.go file contains a syntax error,
b.test fails with an error message. But it wasn't printing
the same FAIL line that a build failure later would print.
This makes all the test failures that happen (once we
decide to start running tests) consistently say FAIL.

Fixes #4701.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13431044