Added tests in int module library as a part of #76268

[ayushmishra2005]

Added Arith Tests library as a part of #76268

r? @matklad

[ayushmishra2005]

@matklad @jyn514 please review this pull request

[matklad]

I think this invokes compiler built-in operations for primitives, rather than going through libcore implementation (rather, ops::Add and friends in libcore are defined in terms of operations on promitives).

So, these tests actually exercise the compiler itself, and not the stdlib, and better be left in place.

@jyn514 I've noticed that a fair share of #76268 PRs now hit this "not actually stdlib" test. This probably means majority of the library tests are now in their proper place and that we can close that issue?

[jyn514]

@matklad there are probably still some left, but I'm ok with closing the issue and moving the info to the dev-guide, maybe. I think #79038 should be either merged or closed before closing #76268 though.

[jyn514]

Closing this since these are testing the compiler and not the standard library.

[jyn514]

As a general rule, if the test would work with #![no_core], it's always a test of the compiler and not the standard library.