Backslash in Windows path is not escaped

Created by: jbeigel

When using tasty-discover on Windows, the path is not properly escaped in Test.Tasty.Discover.generateTestDriver: The value for src is passed from the outside (when tasty-discover is used as preprocessor), but a string is build up from that by simple concatenation, which can result in output like

pure $ T.testGroup "test\Tasty.hs" [...

This leads to a build error "lexical error in string/character literal at character 'T'"