Adds coverage for inject, run, report, and write_results methods and fixes default results file writing on Windows

[lkhudisman]

Add thorough tests for Axe class and fix default results file path

I've added a bunch of new tests for the Axe class to make sure all the main methods behave correctly:

• inject() now has tests to check that it executes the script properly and raises an error if the script is missing.
• run() has tests for different combinations of context and options, plus error handling when the JS fails.
• report() is tested to ensure the output matches expectations for both populated and empty violation lists.
• write_results() now has tests for default filename usage, invalid paths, and unserializable data.

Also fixed an issue in write_results() where Windows was failing because os.getcwd() was not used correctly to get the current working directory. Now it writes the default results.json file as expected across platforms.