Added allow-expect-in-tests / allow-unwrap-in-tests (#2328)

This PR:
* Added the clippy.toml to configure allowable expect / unwrap usage in
tests
* Removed as many expect/allow lines as possible from tests
* moved a bunch of allows to expects where possible

Note: in integration tests, non `#[test]` helper functions are not
covered by this so we had to leave a few lingering `expect(expect_used`
checks around

codex-rs/apply-patch/src/parser.rs

@@ -427,7 +427,6 @@ fn parse_update_file_chunk(
 }
 
 #[test]
-#[allow(clippy::unwrap_used)]
 fn test_parse_patch() {
     assert_eq!(
         parse_patch_text("bad", ParseMode::Strict),