[ELF][test] Rename tail-merge-string-align2.s to merge-string-align2.s

This test was added by D64200/r365139 to check we don't merge
SHF_MERGE|SHF_STRINGS sections with different alignments (that wastes
space and can make MergeTailAlignment::Builder out of sync).

It has nothing to do with tail merge (-O2), so rename it.

llvm-svn: 365442