Use tests module for running tests

[orhun]

Also fixes this error:

error[E0433]: failed to resolve: use of undeclared crate or module `io`
   --> src/main.rs:332:20
    |
332 |     let bytes_in = io::Cursor::new(&include_bytes!("../hello_usb.elf")[..]);
    |                    ^^ use of undeclared crate or module `io`
    |
help: consider importing one of these items
    |
1   + use std::collections::btree_map::Cursor;
    |
1   + use std::collections::linked_list::Cursor;
    |
1   + use std::io::Cursor;
    |
help: if you import `Cursor`, refer to it directly
    |
332 -     let bytes_in = io::Cursor::new(&include_bytes!("../hello_usb.elf")[..]);
332 +     let bytes_in = Cursor::new(&include_bytes!("../hello_usb.elf")[..]);
    |

error[E0433]: failed to resolve: use of undeclared crate or module `io`
   --> src/main.rs:345:20
    |
345 |     let bytes_in = io::Cursor::new(&include_bytes!("../hello_serial.elf")[..]);
    |                    ^^ use of undeclared crate or module `io`
    |
help: consider importing one of these items
    |
1   + use std::collections::btree_map::Cursor;
    |
1   + use std::collections::linked_list::Cursor;
    |
1   + use std::io::Cursor;
    |
help: if you import `Cursor`, refer to it directly
    |
345 -     let bytes_in = io::Cursor::new(&include_bytes!("../hello_serial.elf")[..]);
345 +     let bytes_in = Cursor::new(&include_bytes!("../hello_serial.elf")[..]);
    |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `elf2uf2-rs` (bin "elf2uf2-rs" test) due to 2 previous errors