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>

src/cmd/doc/doc_test.go

@@ -7,7 +7,6 @@ package main
 import (
 	"bytes"
 	"flag"
-	"go/build"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -16,6 +15,14 @@ import (
 	"testing"
 )
 
+func TestMain(m *testing.M) {
+	// otherwise the tests are brittle, as they may give unexpected
+	// output or errors when a suffix match with GOPATH takes place
+	buildCtx.GOPATH = ""
+	dirsInit()
+	os.Exit(m.Run())
+}
+
 func maybeSkip(t *testing.T) {
 	if strings.HasPrefix(runtime.GOOS, "nacl") {
 		t.Skip("nacl does not have a full file tree")
@@ -653,7 +660,7 @@ func TestDotSlashLookup(t *testing.T) {
 			t.Fatal(err)
 		}
 	}()
-	if err := os.Chdir(filepath.Join(build.Default.GOROOT, "src", "text")); err != nil {
+	if err := os.Chdir(filepath.Join(buildCtx.GOROOT, "src", "text")); err != nil {
 		t.Fatal(err)
 	}
 	var b bytes.Buffer