Add tests in mod.rs

[RagnarokMew]

Added 2 tests for calculate_total_file_size(), creating files with certain sizes and testing if the function correctly calculates their sizes.

This PR is targeted at issue #46

TODO

• Use cargo fmt
• Add necessary tests
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[RagnarokMew]

Wanted to also write a test for a full hierarchy, but am unsure how to proceed since the size of the directories can vary (at least that's what I think can happen based on each OS this is ran on). The hierarchy itself is simple:

tempdir
-> 100b-file.txt
-> child
--> 100b-file.txt

Testing on this hierarchy in the code below returns 340 bytes;
Deleting the 100b-files, the changed hierarchy would return 140 bytes, while an empty folder returns 60 bytes.
Any help with this would be greatly appreciated.

My test:

#[test]
    fn test_calculate_total_file_size_directory() {
        let dir_parent = assert_fs::TempDir::new().unwrap();
        let path_file = dir_parent.path().join("100B-text.txt");
        let file = File::create(&path_file).expect("failed to create file");

        let mut buff_writer = BufWriter::new(file);

        let buffer = vec![0u8;100];

        buff_writer.write_all(&buffer).expect("failed to write bytes to file");
        buff_writer.flush().expect("failed to write all bytes to file");
        let dir_child_path = dir_parent.path().join("child");
        fs::create_dir(&dir_child_path).expect("failed to create subdirectory");
        let path_file = dir_child_path.join("100B-text.txt");
        let file = File::create(&path_file).expect("failed to create subdirectory file");

        let mut buff_writer = BufWriter::new(file);

        let buffer = vec![0u8;100];
        buff_writer.write_all(&buffer).expect("failed to write bytes to subdirectory file");
        buff_writer.flush().expect("failed to write all bytes to subdirectory file");


        println!("{}",Meta::calculate_total_file_size(dir_parent.path()));

        assert!(Meta::calculate_total_file_size(dir_parent.path()) == 340);
    }