Don't use hardcoded temp file in test_utils

The hardcoded /tmp/test_emptyfile.fits in test_filecontainer_save in utils/tests/test_utils.py has a few problems:
1. The file is owned by the user who created it, meaning the test fails if another user writes it first
2. The file is left behind after the test. Insignificant, but not good practice
3. If tests are run in parrallel - say, by testing different versions at once - then asserting the file exists seems like a potential race condition