[llvm-objcopy] Fix strip-all-gnu test

Originally the test has a bug: in the input YAML `.symtab.dyn` section has a type
repeated twice with different values (SHT_SYMTAB and SHT_NOBITS). YAML parser
took SHT_NOBITS as a type of the section, so the test wasn't checking the desired
case.

This patch changes the test to verify that a section with SHT_SYMTAB type and
SHF_ALLOC flag is not removed when --strip-all-gnu flag present.

Differential Revision: https://reviews.llvm.org/D143086