test: skip some tests on noopt builder

Adds a new build tag "gcflags_noopt" that can be used in test/*.go
tests.

Fixes #27833

Change-Id: I4ea0ccd9e9e58c4639de18645fec81eb24a3a929
Reviewed-on: https://go-review.googlesource.com/136898
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/checkbce.go

-// +build amd64
+// +build amd64,!gcflags_noopt
 // errorcheck -0 -d=ssa/check_bce/debug=3
 
 // Copyright 2016 The Go Authors. All rights reserved.

test/fixedbugs/issue7921.go

+// +build !gcflags_noopt
 // errorcheck -0 -m
 
 // Copyright 2018 The Go Authors. All rights reserved.

test/nosplit.go

-// +build !nacl,!js
+// +build !nacl,!js,!gcflags_noopt
 // run
 
 // Copyright 2014 The Go Authors. All rights reserved.

test/run.go

@@ -356,6 +356,7 @@ func goDirPackages(longdir string, singlefilepkgs bool) ([][]string, error) {
 type context struct {
 	GOOS     string
 	GOARCH   string
+	noOptEnv bool
 }
 
 // shouldTest looks for build tags in a source file and returns
@@ -375,10 +376,13 @@ func shouldTest(src string, goos, goarch string) (ok bool, whyNot string) {
 		if len(line) == 0 || line[0] != '+' {
 			continue
 		}
+		gcFlags := os.Getenv("GO_GCFLAGS")
 		ctxt := &context{
 			GOOS:     goos,
 			GOARCH:   goarch,
+			noOptEnv: strings.Contains(gcFlags, "-N") || strings.Contains(gcFlags, "-l"),
 		}
+
 		words := strings.Fields(line)
 		if words[0] == "+build" {
 			ok := false
@@ -425,6 +429,10 @@ func (ctxt *context) match(name string) bool {
 		return true
 	}
 
+	if ctxt.noOptEnv && name == "gcflags_noopt" {
+		return true
+	}
+
 	if name == "test_run" {
 		return true
 	}