cmd/go: rename renamed testing import

_go_testing is equally unlikely and much less concerning
than testing_xxxxxxxxxxxx if it appears in an error message
(as it does, for example, in https://storage.googleapis.com/go-build-log/0d543f89/linux-amd64_3467a10e.log).

Change-Id: I45dc3ebe2d3b6c9e53273cd21782ee11a53f5edb
Reviewed-on: https://go-review.googlesource.com/c/go/+/177197
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/load/test.go

@@ -626,8 +626,7 @@ func checkTestFunc(fn *ast.FuncDecl, arg string) error {
 var testinginitTmpl = lazytemplate.New("init", `
 package {{.Name}}
 
-{{/* Avoid a name collision with a name "testing" in user code. */}}
-import testing_xxxxxxxxxxxx "testing"
+import _go_testing "testing"
 
 {{/*
 Call testing.Init before any other user initialization code runs.
@@ -636,7 +635,7 @@ This provides the illusion of the old behavior where testing flags
 were registered as part of the testing package's initialization.
 */}}
 var _ = func() bool {
-	testing_xxxxxxxxxxxx.Init()
+	_go_testing.Init()
 	return true
 }()
 `)