fix mask and tests for avx512

Issue/feature addressed

avx512 backend of tinySIMD had some bugs related to the mask type and testing

Proposed solution

• fixed mask type
• fixed tests

Notes

this PR supersedes !1249 (closed)

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Newly added files are correctly formatted.
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).