part of PR 340 adding test_mount_to_container and separate rootfs file

[tommady]

as requested by separating #340 into small PRs

this part is adding:

1. test_mount_to_container
2. separate rootfs.rs into pieces

please help review 🙇🏻

[utam0k]

It's time to create a rootfs directory and separate the files.

[tommady]

the only logic I modified at here
is because referenced to runC at here

runcC is using device type == bind as the condition.

[codecov-commenter]

Codecov Report

Merging #365 (678b7f9) into main (68d2c0b) will increase coverage by 74.73%.
The diff coverage is 77.84%.

@@            Coverage Diff            @@
##           main     #365       +/-   ##
=========================================
+ Coverage      0   74.73%   +74.73%     
=========================================
  Files         0       51       +51     
  Lines         0     7763     +7763     
=========================================
+ Hits          0     5802     +5802     
- Misses        0     1961     +1961     

[utam0k]

@tommady Instead of creatting an exclusive file for test, it is more natural in Rust to separate some functions into appropriate files based on some criteria, and put the tests associated with the functions into the same file.

[utam0k]

Awesome