ServeFromSubPath: Redirect to URL with subpath when subpath missing (#66724)

* ServeFromSubPath: Redirect to URL with subpath when subpath missing

* Review fixes

* Added tests

* Use constant

* change to useMiddleware

* Update pkg/middleware/subpath_redirect.go

---------

Co-authored-by: Carl Bergquist <carl.bergquist@gmail.com>

pkg/middleware/testing.go

@@ -99,7 +99,10 @@ func (sc *scenarioContext) fakeReqWithParams(method, url string, queryParams map
 	for k, v := range queryParams {
 		q.Add(k, v)
 	}
+
 	req.URL.RawQuery = q.Encode()
+	req.RequestURI = req.URL.RequestURI()
+
 	require.NoError(sc.t, err)
 
 	reqCtx := &contextmodel.ReqContext{