cmd/doc: use empty GOPATH when running the tests

cmd/doc: use empty GOPATH when running the tests

Otherwise, a populated GOPATH might result in failures such as:

	$ go test
	[...] no buildable Go source files in [...]/gopherjs/compiler/natives/src/crypto/rand
	exit status 1

Move the initialization of the dirs walker out of the init func, so that
we can control its behavior in the tests.

Updates #24464.

Change-Id: I4b26a7d3d6809bdd8e9b6b0556d566e7855f80fe
Reviewed-on: https://go-review.googlesource.com/101836
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>