tests: web-server: fix an issue with the web server cache

On a decently fast system, some files may be fetched, modified and then
fetched again within the same second. In that case, the web server
replies with a code 304 ("Not modified") to the 2nd query, causing some
tests to fail.

This commit forces the web server to ignore `If-Modified-Since` HTTP
headers, effectively disabling caching in order to mitigate the problem.

tests/web-server.py

@@ -37,6 +37,7 @@ class RequestHandler(http_server.SimpleHTTPRequestHandler):
     def do_GET(self):
         if self.handle_tokens():
             return None
+        self.headers.__delitem__("If-Modified-Since")
         return super().do_GET()
 
 def run(dir):