df: add unit tests for internal helper functions

[jfinkels]

Unlike other utilities like head or seq, the output of the df program may be different in each environment in which it is run, and even each time it is run (since filesystem usage may change rapidly). This makes it difficult to produce black box tests, as in the tests/by-utils/test_*.rs files. But we can still provide tests for the internal components of df using mock data. This pull request starts to address this by adding unit tests in the df.rs file for internal helper functions mount_info_lt(), is_best(), is_included(), and filter_mount_list().