[llvm-objcopy][test] Fix test that could have passed spuriously

The test was showing that when --strip-unneeded is specified for an
executable, all the symbols are stripped. However, the set of symbols
used in the test would be stripped by --strip-unneeded for an ET_REL
object too. Fix this by adding additional symbols that aren't normally
stripped by --strip-unneeded.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D97664