chore: fix test server to disable cache (microsoft#282)

If pages are cached, then Firefox uses back-forward cache aggressively
to serve the history navigations.

Fixes microsoft/playwright#3693

Author: aslushnikov

tests/async/test_navigation.py

@@ -864,7 +864,6 @@ async def test_wait_for_load_state_should_resolve_after_popup_load(context, serv
     assert popup.url == server.PREFIX + "/one-style.html"
 
 
-@pytest.mark.skip_browser("firefox")
 async def test_go_back_should_work(page, server):
     assert await page.goBack() is None
 

tests/server.py

@@ -112,16 +112,16 @@ def process(self):
                     file_content = (
                         static_path / request.path.decode()[1:]
                     ).read_bytes()
-                except (FileNotFoundError, IsADirectoryError):
-                    request.setResponseCode(HTTPStatus.NOT_FOUND)
-                if file_content:
-                    request.setHeader("Content-Type", mimetypes.guess_type(uri)[0])
+                    request.setHeader(b"Content-Type", mimetypes.guess_type(uri)[0])
+                    request.setHeader(b"Cache-Control", "no-cache, no-store")
                     if uri in gzip_routes:
                         request.setHeader("Content-Encoding", "gzip")
                         request.write(gzip.compress(file_content))
                     else:
                         request.write(file_content)
                     self.setResponseCode(HTTPStatus.OK)
+                except (FileNotFoundError, IsADirectoryError):
+                    request.setResponseCode(HTTPStatus.NOT_FOUND)
                 self.finish()
 
         class MyHttp(http.HTTPChannel):