tests: elfstrmerge warn about STT_SECTION symbol for shstrhndx.

Old linkers might have created an STT_SECTION symbol for the section
header string table section, which isn't actually used.  For now just
warn about such symbols. If such a symbol would actually really be used
(but why?) then we would have to handle it by removing it and rewriting
the symbol table.

This is a testsuite only change, but includes an extra test with files
that have such STT_SECTION symbols to make sure it will be handled in
case we "upgrade" the elfstrmerge test to a real utility.

Signed-off-by: Mark Wielaard <mjw@redhat.com>