Start SQL Server using a synchronous command to avoid intermittent login failures

See https://github.com/doctrine/dbal/pull/2617#discussion_r177250225.

.appveyor.yml

@@ -92,7 +92,7 @@ before_test:
   - ps: >-
       if ($env:db -eq "mssql") {
         $instanceName = $env:db_version.ToUpper()
-        Start-Service "MSSQL`$$instanceName"
+        net start "MSSQL`$$instanceName"
       }
 
 test_script: