tests: check for stderr presences, not exact text

Changes in libcurl 8.3.0 make some testcases fail due to different error
message wording. Adjust testcases so only the presence of an stderr is
verified, not the wording in detail.

Fixes #239

tests.json

@@ -1488,7 +1488,7 @@
                 }
             ],
             "returncode": 9,
-            "stderr": "trurl error: No host part in the URL []\ntrurl error: Try trurl -h for help\n"
+            "stderr": true
         }
     },
     {
@@ -1744,7 +1744,7 @@
         "expected": {
             "stdout": "emanuele6://curl.se/trurl\nhttps://example.org/\n",
             "returncode": 0,
-            "stderr": "trurl note: No host part in the URL []\n"
+            "stderr": true
         }
     },
     {