fix(tests): Adapt CmdlineSuite to default to non-pretty output

This commit adapts the `CmdlineSuite` runner to handle the new pretty-by-default behavior.

The `parse_args` helper in `mypy/test/testcmdline.py` is modified to inject the `--no-pretty` flag into the arguments parsed from '# cmd:' lines in test cases.

This ensures that command-line-driven tests, which run in a separate subprocess and bypass other test helpers, are also executed in non-pretty mode by default.

Part of #19108.

Author: null-dreams

mypy/test/testcmdline.py

@@ -131,8 +131,13 @@ def parse_args(line: str) -> list[str]:
     """
     m = re.match("# cmd: mypy (.*)$", line)
     if not m:
-        return []  # No args; mypy will spit out an error.
-    return m.group(1).split()
+        return ['--no-pretty']  # No args; mypy will spit out an error.
+    args = m.group(1).split()
+    
+    if '--pretty' not in args:
+        args.append('--no-pretty')
+
+    return args
 
 
 def parse_cwd(line: str) -> str | None: