strip: Make sure old .shstrab is removed when eu-strip recreates it.

strip: Make sure old .shstrab is removed when eu-strip recreates it.

Although we always recreate the .shstrtab section for the new output
file we never explicitly assumed it could be removed. It might not be
possible to remove it when the section string table is shared with
a symbol table. But if it is removable we should (and recreate it for
the new section list).

Regression introduced in commit elfutils-0.163-33-gdf7dfab.
"Handle merged strtab/shstrtab string tables in strip and unstrip."
Add extra testcase to explicitly check for this case.

https://sourceware.org/bugzilla/show_bug.cgi?id=21525

Signed-off-by: Mark Wielaard <mark@klomp.org>