[AVR][NFC] Simplify branch relaxation tests (#131871)

Let's use the `.space` directive to simplify AVR's branch relaxation
tests, as noticed by @MaskRay at
#118015 (comment).

Having `.space 4100` instead of `.space 4096` is little bit awkward, but
I'd rather avoid changing two things at once (i.e. current assertions
are fine).

Author: Patryk27