Clean up unit tests in the `dd` crate to make them easier to
manage. This commit does a few things.
* move test cases that test the complete functionality of the `dd`
program from the `dd_unit_tests` module up to the
`tests/by-util/test_dd.rs` module so that they can take advantage of
the testing framework and common testing tools provided by uutils,
* move test cases that test internal functions of the `dd`
implementation into the `tests` module within `dd.rs` so that they
live closer to the code they are testing,
* replace test cases defined by macros with test cases defined by
plain old functions to make the test cases easier to read at a
glance.
