Fix multiprocessing coverage

[Wenzel]

Name                   Stmts   Miss Branch BrPart  Cover
--------------------------------------------------------
checksec/__init__.py       0      0      0      0   100%
checksec/__main__.py      93     93     40      0     0%
checksec/binary.py        19      7      4      1    57%
checksec/elf.py          150     87     38      0    34%
checksec/errors.py         6      2      0      0    67%
checksec/output.py       213    213     74      0     0%
checksec/pe.py            53     53      6      0     0%
checksec/utils.py         72     57     32      0    14%
--------------------------------------------------------
TOTAL                    606    512    194      1    12%

Coverage very low, and even pe.py is at 0%, which makes no sense
The fix --concurrency=multiprocessing doesn't seem to work.

Related: https://coverage.readthedocs.io/en/coverage-5.1/subprocess.html