.travis.yml, tests: Update for GO111MODULE=on [ci-skip]

I've flagged this as "[ci-skip]" becausee it finds a legitimate
regression; hold off on fixing that for another commit.

.travis.yml

@@ -7,10 +7,16 @@ matrix:
     - go: "1.10"
     - go: "1.11"
       env: {GO111MODULE: "off"}
+    - go: "1.11"
+      env: {GO111MODULE: "on"}
     - go: "1.12"
       env: {GO111MODULE: "off"}
+    - go: "1.12"
+      env: {GO111MODULE: "on"}
     - go: "tip"
       env: {GO111MODULE: "off"}
+    - go: "tip"
+      env: {GO111MODULE: "on"}
 
 install: |
   if test -z "$(go env GOMOD)"; then

internal/errcheck/errcheck_test.go

@@ -131,6 +131,9 @@ package custom
 		os.RemoveAll(tmpGopath)
 	}()
 
+	if err := ioutil.WriteFile(path.Join(testBuildTagsDir, "go.mod"), []byte("module github.com/testbuildtags"), 0644); err != nil {
+		t.Fatalf("Failed to write testbuildtags go.mod: %v", err)
+	}
 	if err := ioutil.WriteFile(path.Join(testBuildTagsDir, "custom1.go"), []byte(testBuildCustom1Tag), 0644); err != nil {
 		t.Fatalf("Failed to write testbuildtags custom1: %v", err)
 	}
@@ -167,7 +170,8 @@ package custom
 		checker.Tags = currCase.tags
 
 		loadPackages = func(cfg *packages.Config, paths ...string) ([]*packages.Package, error) {
-			cfg.Env = append(os.Environ(), "GOPATH="+tmpGopath)
+			cfg.Env = append(os.Environ(),
+				"GOPATH="+tmpGopath)
 			cfg.Dir = testBuildTagsDir
 			pkgs, err := packages.Load(cfg, paths...)
 			return pkgs, err
@@ -198,6 +202,10 @@ func TestWhitelist(t *testing.T) {
 }
 
 func TestIgnore(t *testing.T) {
+	const testVendorGoMod = `module github.com/testvendor
+
+require github.com/testlog v0.0.0
+`
 	const testVendorMain = `
 	package main
 
@@ -227,6 +235,9 @@ func TestIgnore(t *testing.T) {
 		os.RemoveAll(tmpGopath)
 	}()
 
+	if err := ioutil.WriteFile(path.Join(testVendorDir, "go.mod"), []byte(testVendorGoMod), 0755); err != nil {
+		t.Fatalf("Failed to write testvendor go.mod: %v", err)
+	}
 	if err := ioutil.WriteFile(path.Join(testVendorDir, "main.go"), []byte(testVendorMain), 0755); err != nil {
 		t.Fatalf("Failed to write testvendor main: %v", err)
 	}
@@ -264,7 +275,9 @@ func TestIgnore(t *testing.T) {
 		checker := NewChecker()
 		checker.Ignore = currCase.ignore
 		loadPackages = func(cfg *packages.Config, paths ...string) ([]*packages.Package, error) {
-			cfg.Env = append(os.Environ(), "GOPATH="+tmpGopath)
+			cfg.Env = append(os.Environ(),
+				"GOPATH="+tmpGopath,
+				"GOFLAGS=-mod=vendor")
 			cfg.Dir = testVendorDir
 			pkgs, err := packages.Load(cfg, paths...)
 			return pkgs, err
@@ -291,6 +304,10 @@ func TestIgnore(t *testing.T) {
 }
 
 func TestWithoutGeneratedCode(t *testing.T) {
+	const testVendorGoMod = `module github.com/testvendor
+
+require github.com/testlog v0.0.0
+`
 	const testVendorMain = `
 	// Code generated by protoc-gen-go. DO NOT EDIT.
 	package main
@@ -321,6 +338,9 @@ func TestWithoutGeneratedCode(t *testing.T) {
 		os.RemoveAll(tmpGopath)
 	}()
 
+	if err := ioutil.WriteFile(path.Join(testVendorDir, "go.mod"), []byte(testVendorGoMod), 0755); err != nil {
+		t.Fatalf("Failed to write testvendor go.mod: %v", err)
+	}
 	if err := ioutil.WriteFile(path.Join(testVendorDir, "main.go"), []byte(testVendorMain), 0755); err != nil {
 		t.Fatalf("Failed to write testvendor main: %v", err)
 	}
@@ -351,7 +371,9 @@ func TestWithoutGeneratedCode(t *testing.T) {
 		checker := NewChecker()
 		checker.WithoutGeneratedCode = currCase.withoutGeneratedCode
 		loadPackages = func(cfg *packages.Config, paths ...string) ([]*packages.Package, error) {
-			cfg.Env = append(os.Environ(), "GOPATH="+tmpGopath)
+			cfg.Env = append(os.Environ(),
+				"GOPATH="+tmpGopath,
+				"GOFLAGS=-mod=vendor")
 			cfg.Dir = testVendorDir
 			pkgs, err := packages.Load(cfg, paths...)
 			return pkgs, err