Ignore files generated by `make check`

[J-Lentz]

Description Add a .gitignore file for each unit test, so that Git ignores all files generated by make check.

How Has This Been Tested?

make
make check -k
git status

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules
• [x] New check tests, if applicable, are included
• [x] make distcheck passes