[llvm-exegis][test] Ignore file path in no-aliasing-ld-str.s
This test failed on a buildbot where the path happened to contain
"ld2":
/home/b/sanitizer-aarch64-linux-bootstrap-hwasan/build/llvm_build2_hwasan/test/tools/llvm-exegesis/AArch64/Output/no-aliasing-ld-str.s.tmp.obj: file format elf64-littleaarch64
To fix this, make lit consume a line past this one and
do the check-not after that point.