Rollup merge of #112295 - ForrestOfBarnes:tests-listing-format-json-windows-fix, r=pietroalbini

Fix the tests-listing-format-json test on Windows

tests/ui/test-attrs/tests-listing-json-format.rs was failing on Windows because each path in the json-formatted output contained "\\\\" instead of "\\". `runtest::TestCx::normalize_output` already checks the compile flags for json-related arguments to handle this case, so I added an equivalent check for the new run flag.

Author: matthiaskrgr