Adds a test to ensure that a file copied via the AppCache feature preserves the file's original mtime. 


In #302 I determined that as long as the files copied into the cache dir preserve the original mtime then the FIFO cache cleaning logic that we rely on is valid. This test closes out this comment that asserted we need to verify whether or not the buildpack preserved mtime when copying files #302 (comment).


[#close 302]

Assert that loading from cache does not over-write files.

I previously introduced a commit that had a test intended to prove that we were persisting the mtime of files copied into the cache to close out this comment #302 (comment). The test passed locally, but failed on CI.

It appears the behavior of fs-extra with regards to mtime for copying/moving directories is system dependent. To compensate, I switched to `cp_r` which explicitly is designed to preserve mtime https://crates.io/crates/cp_r. It has ~27,000 lifetime downloads and has activity as recently as a 9 days ago. https://docs.rs/cp_r/latest/cp_r/#release-history

Copying and preserving mtime is the primary reason this library exists. From the docs.rs:

> Copy a directory tree, including mtimes and permissions.

It does not have the ability to skip-overwriting existing files which I need, however it has a generic `filter` closure which allows me to assemble this behavior myself.