fix(tests): Adapt PEP561Suite to default to non-pretty output

This commit adapts the `PEP561Suite` test runner to handle the new pretty-by-default behavior.

The `parse_mypy_args` helper in `mypy/test/testpep561.py` is modified to inject the `--no-pretty` flag into the arguments parsed from '# flags:' lines in test cases.

Part of #19108.

Author: null-dreams

mypy/test/testpep561.py

@@ -174,5 +174,8 @@ def parse_pkgs(comment: str) -> tuple[list[str], list[str]]:
 def parse_mypy_args(line: str) -> list[str]:
     m = re.match("# flags: (.*)$", 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()
+    args.append('--no-pretty')
+
+    return args