test: also confirm str(exc) for invalid requests

tests/requests/invalid/version_03.http

@@ -0,0 +1,2 @@
+GET /foo HTTP/0.9\r\n
+\r\n

tests/requests/invalid/version_03.py

@@ -0,0 +1,2 @@
+from gunicorn.http.errors import InvalidHTTPVersion
+request = InvalidHTTPVersion

tests/test_invalid_requests.py

@@ -23,5 +23,6 @@ def test_http_parser(fname):
     cfg = env["cfg"]
     req = treq.badrequest(fname)
 
-    with pytest.raises(expect):
+    # telling pytest to match the exception string validates its __str__()
+    with pytest.raises(expect, match="."):
         req.check(cfg)