[test][ELF] Verify debug line corruption messages are printed once

LLD warns if it encounters malformed debug data when parsing line
information for an undefined reference. We only want to warn once.

This patch adds additional checking to make sure the warnings are
printed only once, both for variables within the same program and
variables in later line programs.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D71759