test: replace assertion error functions w regex err msg

[updated by @refack] Error message changes are semver-major, so it should be a good practice to tighten up the tests to check for it. [end update]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test