Increase test coverage

[janosh]

Coverage is currently at

$ pytest --cov-report term-missing --cov pdf_compressor
---------- coverage: platform darwin, python 3.9.7-final-0 -----------
Name                         Stmts   Miss  Cover   Missing
----------------------------------------------------------
pdf_compressor/__init__.py       3      0   100%
pdf_compressor/ilovepdf.py     116     21    82%   65, 76, 83, 148, 156, 159, 188-198, 210, 233, 250, 261, 275-276, 291
pdf_compressor/main.py          54      4    93%   102, 126-127, 154
pdf_compressor/utils.py         62      9    85%   54, 59, 113, 122-125, 130-134
----------------------------------------------------------
TOTAL                          235     34    86%

utils.py should be easier to test than ilovepdf.py.

[MarcinGladkowski]

On it.