Add an option to lit to make it generate a unique directory for each test #53542
Description
At present, %t for a test in test-dir/test-name ends up expanding to something like test-dir/Output/test-name. This means that tests sharing a test-dir all end up sharing the same Output directory, which makes it tricky to transfer the output directory to a remote machine and back to run a remote test.
It would be good if there was an option, which for compatibility reasons should not be the default, to expand %t instead to test-dir/Output/test-name/test-name — i.e. to give each test its own separate output directory that can't conflict with any other test in the same test-dir. This would make it easy to transfer the contents of the directory (e.g. using rsync) without worrying about other tests creating files that might conflict.
(There is an existing issue, #29636, that seems to also want a unique directory per test. I thought I'd raise this as a new issue because I don't think it's quite the same — but maybe it is, in which case we can close one of them.)