ERT tests should not assume anything about the state

[bcc32]

Some of the newer tests seem to only work when run in the default, alphabetical order with all of the tests selected. For example, ledger-report/expand-format-specifiers depends on the current buffer being some source buffer whose major mode is ledger-mode. This is not a valid assumption for running ERT tests interactively, or running a subset of the tests.

We should:

• Run tests in randomized order to catch such accidental ordering dependencies
• Consistently use ledger-tests-with-temp-file to set up the test state
• Perhaps add an AGENTS.md file to ensure this doesn't break going forward?