tests/test_parsing_expressions: more specific assert statements

[stanislaw]

Setting the exact string match expectations helps to visualize what exactly changes with any attempts to refactor the error reporting functionality.

Code review checklist

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated

Doesn't seem to be applicable:

• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• Changelog(s) has/have been updated, as needed (see CHANGELOG.md, no need
  to update for typo fixes and such).

[igordejanovic]

Looks good.