Fix interactive running of ERT tests

[bcc32]

This PR consists of a few commits cleaning up some issues with running the ledger ERT tests interactively. Some of them are still a little unreliable, but this PR makes the following improvements:

1. Do not depend on the user's customizations, if any. ledger-tests-with-temp-file will now reset ledger customizations to the actual defaults and not the user's customized value. (This will also affect the user's session, if they're running the ledger tests in their regular Emacs, but that is hard to avoid.)
2. Fix some macro hygiene issues with ledger-tests-with-temp-file
3. Fix some incorrect assumptions about the selected window prior to the test beginning to run.