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

This commit modifies the `test_error_stream` runner to set `options.pretty = False`, ensuring its tests continue to pass with the new pretty-by-default behavior.

Part of #19108.

Author: null-dreams

mypy/test/testerrorstream.py

@@ -25,6 +25,7 @@ def test_error_stream(testcase: DataDrivenTestCase) -> None:
     The argument contains the description of the test case.
     """
     options = Options()
+    options.pretty = False
     options.show_traceback = True
     options.hide_error_codes = True