[dexter] Change line label reference syntax to enable label-relative offsets (2/2)

Update dexter tests to use the new line label reference syntax introduced in D101147.

Updated with:
https://gist.github.com/OCHyams/8255efe7757cac266440ed2ba55f1442

Reviewed By: chrisjackson, jmorse

Differential Revision: https://reviews.llvm.org/D101148