Improve expectation functions

[nealrichardson]

If the expectations fail, they fail in messy ways. expect_header() gives a message about improper ... (some sort of quasiquotation issue inside testthat), and expect_VERB() seems to Error completely rather than just fail. They both are probably connected to testthat 3.0 internal NSE changes. Maybe we can raise custom conditions and write custom expectation functions to catch them.