Review replacing expectedFailure decorator with assertRaisesRegex

# ✨ Feature Request

Currently we make use of the @unittest.expectedFailure decorator rather than, say, [​assertRaisesRegex](https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaisesRegex), which would allow the exception message to be checked (It looks like there is no option to do this with [​expectedFailure](https://docs.python.org/3/library/unittest.html#unittest.expectedFailure)?)

There are currently 11 xfailed tests, so there are only 11 tests that would need updating.

## Requirements

Expected failures to be captured explicitly.

## Context

Simplifying the unittesting so expectedFailures are testing for known, expected, failures.

Search the test suite for the 11 uses of unittest.expectedFailure and inspect each expected failure before changing it. Replace the implicit expected-failure handling with explicit exception assertions that check the message, then run the relevant tests and the full test suite to confirm all expected failures are captured explicitly.

Written by the indexing model from the issue text.