cmd/dist, os/user: test os/user in osusergo mode as well, fix plan9 & windows

Would've caught two regressions so far, and found two more.

Updates #24841
Updates #24845 (package net remains)

Change-Id: I57ad06eb54e04b8c99b5d2e7f24c77ad865224e8
Reviewed-on: https://go-review.googlesource.com/107300Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -407,6 +407,18 @@ func (t *tester) registerTests() {
 		}
 	}
 
+	// Test the os/user package in the pure-Go mode too.
+	if !t.compileOnly {
+		t.tests = append(t.tests, distTest{
+			name:    "osusergo",
+			heading: "os/user with tag osusergo",
+			fn: func(dt *distTest) error {
+				t.addCmd(dt, "src", t.goTest(), t.timeout(300), "-tags=osusergo", "os/user")
+				return nil
+			},
+		})
+	}
+
 	if t.race {
 		return
 	}

src/os/user/lookup_stubs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !cgo,!windows,!plan9 android osusergo
+// +build !cgo,!windows,!plan9 android osusergo,!windows,!plan9
 
 package user
 

src/os/user/user_test.go

@@ -5,6 +5,8 @@
 package user
 
 import (
+	"internal/testenv"
+	"os"
 	"runtime"
 	"testing"
 )
@@ -16,6 +18,20 @@ func checkUser(t *testing.T) {
 }
 
 func TestCurrent(t *testing.T) {
+	// The Go builders (in particular the ones using containers)
+	// often have minimal environments without $HOME or $USER set,
+	// which breaks Current which relies on those working as a
+	// fallback.
+	// TODO: we should fix that (Issue 24884) and remove these
+	// workarounds.
+	if testenv.Builder() != "" && runtime.GOOS != "windows" && runtime.GOOS != "plan9" {
+		if os.Getenv("HOME") == "" {
+			os.Setenv("HOME", "/tmp")
+		}
+		if os.Getenv("USER") == "" {
+			os.Setenv("USER", "gobuilder")
+		}
+	}
 	u, err := Current()
 	if err != nil {
 		t.Fatalf("Current: %v (got %#v)", err, u)