Use sentinel error rather than string matching in GEOS tests

[peterstace]

Description

This makes the tests more robust to changes in the error messages, and also provides a code link back to the checked error for readers of the tests.

Check List

Have you:

• Added unit tests? Yes.

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) N/A, minor internal change.

Related Issue

• N/A

[peterstace]

Thanks Albert!