[llvm] [AVR] Simplify branch relaxation tests (PR #131871)

[Ben Shi via llvm-commits]

https://github.com/benshi001 approved this pull request.

LGTM! But it would be better to change the title to `[AVR][NFC] Simplify branch relaxation tests`.

https://github.com/llvm/llvm-project/pull/131871