Tests for pdfwf

Test files should take the form *_test.py and tests inside files should be top-level functions named test_*().

Tests should typically:

• test a single aspect of the code (e.g., to test feature a and b, use two separate test functions),
• only test the interface (e.g., tests should not check internal implementation details), and
• tests should not rely on the order in which they are executed.