Move the filesystem customization tests into their own file.
Additionally add tests for unmarshalling filesystem customizations
from TOML, since we have added a new `minsize` tag to ensure
consistency with the json tag.
The new tests check the validation of the TOML input and ensures
that either one or both of the `minsize` and `size` inputs are
set. If both are set, the input is checked to ensure that both
match.
