Truffle default account private keys

If they are internal functions you can inherit them and test them, if they are private functions, I believe the only way to unit test them is to make them public/internal, test them and then change them back to private once the tests are passing.

Another way to go about it would be to refactor your code so that the private functions are part of a library that you import into the contract. That way the functions are public in the library but not exposed in the actual contract.

Ganache allocates by default 10 accounts for private keys 0x000...1 thru 0x000...a.

You obviously don't want to use these accounts in your actual deployment, because everybody knows these private keys, they are most likely continuously monitored, and whenever somebody transfers ether into any of them, it is transferred out of them sooner than later.

You can override this configuration by stating your desired set of private keys, for example:

ganache-cli
    --account=0x00000000000000000000000000000000000000000000000000000000deadbeef,99999999999
    --account=0x00000000000000000000000000000000000000000000000000000000cafebabe,99999999999

But for testing purpose, there is no reason not to use the default set of private keys.

That is, unless you need more than 10 accounts in any of your tests.