testsuite: An overhaul of the driver

This tries to put the testsuite driver into a slightly more maintainable condition:

• Add type annotations where easily done
• Use pathlib.Path instead of str paths
• Make it pass the mypy typechecker
• Add lint step to CI to enforce typechecker

It also fixes a few issues in the JUnit output:

• Make it pass mypy
• Fix a typo in test name field
• Report more stderr output
• Report stdout output

The JUnit issues were, in fact, all I set out to fix when I went down this road. However, as you can see, things quickly snowballed.