tests: add pytest tests for ukify

tests: add pytest tests for ukify

Some gymnastics were needed to import ukify as a module. Before the file
was templated, this was trivial: insert the directory in sys.path, call import.
But it's a real pain to import the unsuffixed file after processing. Instead,
the untemplated file is imported, which works well enough for tests and is
very simple.

The tests can be called via pytest:
  PATH=build/:$PATH pytest -v src/ukify/test/test_ukify.py
or directly:
  PATH=build/:$PATH src/ukify/test/test_ukify.py
or via the meson test machinery output:
  meson test -C build test-ukify -v
or without verbose output:
  meson test -C build test-ukify

Zekret files are obfuscated using base64.