Rollup merge of #138546 - GuillaumeGomez:integer-to-string-tests, r=Amanieu

Add integer to string formatting tests

As discussed in rust-lang/rust#136264, there doesn't seem to have tests to ensure that int to string conversion is performed correctly, only sporadic tests here and there. Now we have some basic tests. :)

r? `````@Mark-Simulacrum`````

Author: Zalathar