tests: use another url because jmeter-plugins.org one has issues with Tornado's simple HTTP client.

tests/test_httpheader.py

@@ -8,7 +8,7 @@ pytestmark = pytest.mark.asyncio
 async def test_redirection(get_version):
     assert await get_version("jmeter-plugins-manager", {
         "source": "httpheader",
-        "url": "https://jmeter-plugins.org/get/",
-        "regex": r'/([\d.]+)/',
-    }) == "1.6"
+        "url": "https://www.unifiedremote.com/download/linux-x64-deb",
+        "regex": r'urserver-([\d.]+).deb',
+    }) != None