go/packages: skip TestLoadImportsC when Go has been built without cgo

This test is unsurprisingly failing on the nocgo builder because
the cgo packages don't exist on those builders.

Updates golang/go#28040

Change-Id: I633b73bb48e76824645e4e8dd141fb42c9adc19f
Reviewed-on: https://go-review.googlesource.com/c/140121
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/packages/packages_test.go

@@ -282,52 +282,6 @@ func TestLoadImportsTestVariants(t *testing.T) {
 	}
 }
 
-func TestLoadImportsC(t *testing.T) {
-	// This test checks that when a package depends on the
-	// test variant of "syscall", "unsafe", or "runtime/cgo", that dependency
-	// is not removed when those packages are added when it imports "C".
-	//
-	// For this test to work, the external test of syscall must have a dependency
-	// on net, and net must import "syscall" and "C".
-	if runtime.GOOS == "windows" {
-		t.Skipf("skipping on windows; packages on windows do not satisfy conditions for test.")
-	}
-	if runtime.GOOS == "plan9" {
-		// See https://github.com/golang/go/issues/27100.
-		t.Skip(`skipping on plan9; for some reason "net [syscall.test]" is not loaded`)
-	}
-
-	cfg := &packages.Config{
-		Mode:  packages.LoadImports,
-		Tests: true,
-	}
-	initial, err := packages.Load(cfg, "syscall", "net")
-	if err != nil {
-		t.Fatalf("failed to load imports: %v", err)
-	}
-
-	_, all := importGraph(initial)
-
-	for _, test := range []struct {
-		pattern    string
-		wantImport string // an import to check for
-	}{
-		{"net", "syscall:syscall"},
-		{"net [syscall.test]", "syscall:syscall [syscall.test]"},
-		{"syscall_test [syscall.test]", "net:net [syscall.test]"},
-	} {
-		// Test the import paths.
-		pkg := all[test.pattern]
-		if pkg == nil {
-			t.Errorf("package %q not loaded", test.pattern)
-			continue
-		}
-		if imports := strings.Join(imports(pkg), " "); !strings.Contains(imports, test.wantImport) {
-			t.Errorf("package %q: got \n%s, \nwant to have %s", test.pattern, imports, test.wantImport)
-		}
-	}
-}
-
 func TestLoadAbsolutePath(t *testing.T) {
 	tmp, cleanup := makeTree(t, map[string]string{
 		"gopatha/src/a/a.go": `package a`,

go/packages/packagescgo_test.go

@@ -0,0 +1,60 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo
+
+package packages_test
+
+import (
+	"golang.org/x/tools/go/packages"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestLoadImportsC(t *testing.T) {
+	// This test checks that when a package depends on the
+	// test variant of "syscall", "unsafe", or "runtime/cgo", that dependency
+	// is not removed when those packages are added when it imports "C".
+	//
+	// For this test to work, the external test of syscall must have a dependency
+	// on net, and net must import "syscall" and "C".
+	if runtime.GOOS == "windows" {
+		t.Skipf("skipping on windows; packages on windows do not satisfy conditions for test.")
+	}
+	if runtime.GOOS == "plan9" {
+		// See https://github.com/golang/go/issues/27100.
+		t.Skip(`skipping on plan9; for some reason "net [syscall.test]" is not loaded`)
+	}
+
+	cfg := &packages.Config{
+		Mode:  packages.LoadImports,
+		Tests: true,
+	}
+	initial, err := packages.Load(cfg, "syscall", "net")
+	if err != nil {
+		t.Fatalf("failed to load imports: %v", err)
+	}
+
+	_, all := importGraph(initial)
+
+	for _, test := range []struct {
+		pattern    string
+		wantImport string // an import to check for
+	}{
+		{"net", "syscall:syscall"},
+		{"net [syscall.test]", "syscall:syscall [syscall.test]"},
+		{"syscall_test [syscall.test]", "net:net [syscall.test]"},
+	} {
+		// Test the import paths.
+		pkg := all[test.pattern]
+		if pkg == nil {
+			t.Errorf("package %q not loaded", test.pattern)
+			continue
+		}
+		if imports := strings.Join(imports(pkg), " "); !strings.Contains(imports, test.wantImport) {
+			t.Errorf("package %q: got \n%s, \nwant to have %s", test.pattern, imports, test.wantImport)
+		}
+	}
+}