test: recognize cgo build tag

This requires us to add a fake argument to issue36705.go so that the
test driver will build it with "go run" rather than "go tool compile".

Change-Id: Id08b97d898ee3e9d6c1fbb072a0a9317ed9faedd
Reviewed-on: https://go-review.googlesource.com/c/go/+/304569
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue36705.go

@@ -1,5 +1,5 @@
 // +build cgo
-// run
+// run fake-arg-to-force-use-of-go-run
 
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/run.go

@@ -56,6 +56,7 @@ func defaultAllCodeGen() bool {
 
 var (
 	goos, goarch string
+	cgoEnabled   bool
 
 	// dirs are the directories to look for *.go files in.
 	// TODO(bradfitz): just use all directories?
@@ -82,6 +83,10 @@ func main() {
 
 	goos = getenv("GOOS", runtime.GOOS)
 	goarch = getenv("GOARCH", runtime.GOARCH)
+	cgoEnv, err := exec.Command(goTool(), "env", "CGO_ENABLED").Output()
+	if err == nil {
+		cgoEnabled, _ = strconv.ParseBool(strings.TrimSpace(string(cgoEnv)))
+	}
 
 	findExecCmd()
 
@@ -369,6 +374,7 @@ func goDirPackages(longdir string, singlefilepkgs bool) ([][]string, error) {
 type context struct {
 	GOOS       string
 	GOARCH     string
+	cgoEnabled bool
 	noOptEnv   bool
 }
 
@@ -393,6 +399,7 @@ func shouldTest(src string, goos, goarch string) (ok bool, whyNot string) {
 		ctxt := &context{
 			GOOS:       goos,
 			GOARCH:     goarch,
+			cgoEnabled: cgoEnabled,
 			noOptEnv:   strings.Contains(gcFlags, "-N") || strings.Contains(gcFlags, "-l"),
 		}
 
@@ -448,6 +455,10 @@ func (ctxt *context) match(name string) bool {
 		}
 	}
 
+	if name == "cgo" && ctxt.cgoEnabled {
+		return true
+	}
+
 	if name == ctxt.GOOS || name == ctxt.GOARCH || name == "gc" {
 		return true
 	}