Used this as an opportunity to refactor how some tests were
making assertions about error strings.

New test helpers make it easy for an error string to be expected as an
exact string, as a string built using sprintf, as a regexp, or as a
string built to include the platform-specific x509 error string.

All of these helpers can be used in a single `wantErr` field of a test
table. They can be used for both unit tests and integration tests.

Co-authored-by: Benjamin A. Petersen <ben@benjaminapetersen.me>