test: remove hyphens from test file names

[lobis]

Same as scikit-hep/awkward#2044.

Hyphens in the test names breaks PyCharm for some reason, tests cannot be ran from the IDE.

I can do the renaming but I will wait until people currently working on these files are done to avoid merge conflicts.

The following command can be used to rename the tests files (run from the tests dir):

find . -type f -name "*-*" -exec sh -c 'mv "$0" "${0//-/_}"' {} \;