[llvm-objcopy][test] Improve empty section tests

empty-sections.test: add two tests adapted from @jhenderson's https://reviews.llvm.org/D74755#1882221
strip-non-alloc.test: improve. D74755 will change the attribution of an empty section.

They mostly test the behavior of Object.cpp:sectionWithinSegment : how we attribute sections to segments.
`ParentSegment` can affect some subtle layout decisions.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D74879