We've had a series of problems with tests unexpectedly (and innocently)
looking at system files that appear to (but don't) change in meaningful ways,
like /dev/null on OS X having a modification time set to the current time.

Cut all these off by only applying file change detection to the local package
root: the GOROOT or specific sub-GOPATH in which the package being tested
is found.

(This means that if you test reads /tmp/x and you change /tmp/x, the cached
result will still be used. Don't do that, or else use -count=1.)

Fixes #23390.

Change-Id: I30b6dd194835deb645a040aea5e6e4f68af09edb
Reviewed-on: https://go-review.googlesource.com/87015
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>