runtime: test for cgo build tag in TestGdbPythonCgo

Testing whether cgo is enabled in go/build is not the same as testing
whether the go tool supports cgo. They differ, for example, when using
GOARCH=386 on an amd64 system, as for a cross-build cgo is disabled by default.

Change-Id: Ib59106c92a3131b73ac6a91c0f7658a1769acf73
Reviewed-on: https://go-review.googlesource.com/c/go/+/174098
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/runtime-gdb_test.go

@@ -7,7 +7,6 @@ package runtime_test
 import (
 	"bytes"
 	"fmt"
-	"go/build"
 	"internal/testenv"
 	"io/ioutil"
 	"os"
@@ -140,8 +139,8 @@ func TestGdbPythonCgo(t *testing.T) {
 }
 
 func testGdbPython(t *testing.T, cgo bool) {
-	if cgo && !build.Default.CgoEnabled {
-		t.Skip("skipping because cgo is not enabled")
+	if cgo {
+		testenv.MustHaveCGO(t)
 	}
 
 	checkGdbEnvironment(t)