Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

cmd/dump.go

@@ -203,8 +203,8 @@ func runDump(ctx context.Context, cmd *cli.Command) error {
 			}
 		}()
 
-		targetDBType := cmd.String("database")
-		if len(targetDBType) > 0 && targetDBType != setting.Database.Type.String() {
+		targetDBType := setting.DatabaseType(cmd.String("database"))
+		if targetDBType != "" && targetDBType != setting.Database.Type {
 			log.Info("Dumping database %s => %s...", setting.Database.Type, targetDBType)
 		} else {
 			log.Info("Dumping database...")