MM-16506: conditionally build go.mod for plugins (#11430)

* MM-16506: conditionally build go.mod for plugins

Unless `GO111MODULE=off`, generate a `go.mod` that points at the local copy of `mattermost-server` to ensure plugin tests that compile source code on demand always test with the local copy of mattermost-server.  This also fixes an issue with early adopters of `GO111MODULE=on` on the server failing to find the right version of go-i18n dependencies.

* plugin: enable testlib resource management

* customize fileutils.Find* for testlib

utils/test_files_compiler.go

@@ -4,23 +4,56 @@
 package utils
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
+func goMod(t *testing.T, dir string, args ...string) {
+	cmd := exec.Command("go", append([]string{"mod"}, args...)...)
+	cmd.Dir = dir
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Failed to %s: %s", strings.Join(args, " "), string(output))
+	}
+}
+
 func CompileGo(t *testing.T, sourceCode, outputPath string) {
 	dir, err := ioutil.TempDir(".", "")
 	require.NoError(t, err)
 	defer os.RemoveAll(dir)
-	require.NoError(t, ioutil.WriteFile(filepath.Join(dir, "main.go"), []byte(sourceCode), 0600))
+
+	dir, err = filepath.Abs(dir)
+	require.NoError(t, err)
+
+	// Write out main.go given the source code.
+	main := filepath.Join(dir, "main.go")
+	err = ioutil.WriteFile(main, []byte(sourceCode), 0600)
+	require.NoError(t, err)
+
+	if os.Getenv("GO111MODULE") != "off" {
+		var mattermostServerPath string
+
+		// Generate a go.mod file relying on the local copy of the mattermost-server.
+		// testlib linked mattermost-server into the general temporary directory for this test.
+		mattermostServerPath, err = filepath.Abs("mattermost-server")
+		require.NoError(t, err)
+
+		goMod(t, dir, "init", "mattermost.com/test")
+		goMod(t, dir, "edit", "-require", "github.com/mattermost/mattermost-server@v0.0.0")
+		goMod(t, dir, "edit", "-replace", fmt.Sprintf("github.com/mattermost/mattermost-server@v0.0.0=%s", mattermostServerPath))
+	}
+
 	cmd := exec.Command("go", "build", "-o", outputPath, "main.go")
 	cmd.Dir = dir
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
-	require.NoError(t, cmd.Run(), "failed to compile go")
+	err = cmd.Run()
+	require.NoError(t, err, "failed to compile go")
 }