MM-31082: Use a base path for test assets (#16478)

* MM-31082: Use a base path for test assets

Tests can be run from other tools using a different base path.
We need to respect the MM_SERVER_PATH variable for that.

https://mattermost.atlassian.net/browse/MM-31082

```release-note
NONE
```

* do not append mattermost-server if env var is set

* fix typo

testlib/helper.go

@@ -9,6 +9,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/mattermost/mattermost-server/v5/mlog"
@@ -139,14 +140,28 @@ func (h *MainHelper) setupResources() {
 func (h *MainHelper) PreloadMigrations() {
 	var buf []byte
 	var err error
+	basePath := os.Getenv("MM_SERVER_PATH")
+	relPath := "testlib/testdata"
 	switch *h.Settings.DriverName {
 	case model.DATABASE_DRIVER_POSTGRES:
-		buf, err = ioutil.ReadFile("mattermost-server/testlib/testdata/postgres_migration_warmup.sql")
+		var finalPath string
+		if basePath != "" {
+			finalPath = filepath.Join(basePath, relPath, "postgres_migration_warmup.sql")
+		} else {
+			finalPath = filepath.Join("mattermost-server", relPath, "postgres_migration_warmup.sql")
+		}
+		buf, err = ioutil.ReadFile(finalPath)
 		if err != nil {
 			panic(fmt.Errorf("cannot read file: %v", err))
 		}
 	case model.DATABASE_DRIVER_MYSQL:
-		buf, err = ioutil.ReadFile("mattermost-server/testlib/testdata/mysql_migration_warmup.sql")
+		var finalPath string
+		if basePath != "" {
+			finalPath = filepath.Join(basePath, relPath, "mysql_migration_warmup.sql")
+		} else {
+			finalPath = filepath.Join("mattermost-server", relPath, "mysql_migration_warmup.sql")
+		}
+		buf, err = ioutil.ReadFile(finalPath)
 		if err != nil {
 			panic(fmt.Errorf("cannot read file: %v", err))
 		}