Unit test for filehash.rs

[kiavash-at-work]

This PR includes:

• unit tests for filehash.rs functions.
  • tempfile crate was brought into scope only for unit tests [dev-dependencies]. It is used for generating temp folder in the system temp directory. This temp folder will be removed by OS as soon as its variable goes out of scope.
• removed pub from get_filehash_local() and get_filehash_http_sftp().
  • They are called only by get_filehash() which is already in the scope.
• refactored the error conversation to String from get_filehash_local() and get_filehash_http_sftp() to get_filehash()
  • This move makes it possible to implement unit tests for get_filehash_local() and get_filehash_http_sftp() that checks and verifies the complete error codes.
  • The code is easier to read and follow because now the Err to String conversation happens only once in each branch of get_filehash().
  • The unit test for get_filehash() only verifies the first 5 character of the error message because get_filehash() returns error in String and to avoid variation in error message implementations by the underlying OS.

[kiavash-at-work]

While reviewing all the calls to get_filehash() which are located here and here and here, it seems all
of these references expect Err(Box<dyn Error>) while get_filehash() currently returns Err(String). I didn't change get_filehash() return value but I think it makes sense to change it to Err(Box<dyn Error>). If that is inline with the rest of code error handling, I will update it in the next PR. The change should be minimal

Replace function here with

use std::error::Error;

pub fn get_filehash(file: String, port: u16) -> Result<Vec<u8>, Box<dyn Error>> {
    if file.starts_with("https://") || file.starts_with("http://") || file.starts_with("sftp://") {
        get_filehash_http_sftp(file, port)
            .map_err(|e| e.into())
    } else {
        get_filehash_local(file)
            .map_err(|e| e.into())
    }
}

and unit test here with

                        e.to_string().truncate(truncate_size),

[ashuio]

Thanks for the PR, I'll merge it. Sorry for the delay.