[lld][ELF][test] Add testing for IE/LD TLS weak undef references

Whilst migrating/retiring some downstream testing, I came across a test
for weak undef IE and LD TLS references, but was unable to find any
equivalent in LLD's upstream testing. There does seem to be some slight
subtle differences that could be worth testing compared to LE TLS
references, in particular that IE can be relaxed to LE in this case,
hence this change.

Differential Revision: https://reviews.llvm.org/D95124

Reviewed by: grimar, MaskRay