[llvm-objcopy][llvm-strip][test] Improve testing

This patch adds a number of new test cases that cover various
llvm-objcopy and llvm-strip features that had missing test coverage of
various descriptions:
* --add-section - checked the shdr properties, not just the content.
* Dedicated test case for --add-symbol when there are many sections.
* Show that --change-start accepts negative values without overflow.
  This was previously present but got lost between review versions.
* --dump-section - show that multiple sections can be dumped
  simultaneously to different files, and that an error is reported when
  a section cannot be found.
* --globalize-symbol(s) - show that symbols that are not mentioned are
  not globalized, if they would otherwise be, and that missing symbols
  from the list do not cause problems.
* --keep-global-symbol - show that the --regex option can be used in
  conjunction with this option.
* --keep-symbol - show that the --regex option can be used in
  conjunction with this option.
* --localize-symbol(s) - show that symbols that are not mentioned are
  not localized, if they would otherwise be, and that missing symbols
  from the list do not cause problems.
* --prefix-alloc-sections - show the behaviour of an empty string
  argument and multiple arguments.
* --prefix-symbols - show the behaviour of an empty string argument and
  multiple arguments. Also show the option applies to undefined symbols.
* --redefine-symbol - show that symbols with no name can be renamed,
  that it is not an error if a symbol is not specified, and that the
  option doesn't chain (i.e. --redefine-sym a=b --redefine-sym b=c does
  not redefine a as c).
* --rename-section - show that all section flags are preserved if none
  are specified. Also show that the option does not chain.
* --set-section-alignment - show that only specified sections have
  their alignments changed.
* --set-section-flags - show which section flags are preserved when this
  option is used. Also show that unspecified sections are not affected.
* --preserve-dates - show that -p is an alias of --preserve-dates.
* --strip-symbol - show that --regex works with this option for
  llvm-objcopy as well as llvm-strip.
* --strip-unneeded-symbol(s) - show more clearly that needed symbols are
  not stripped even if requested by this option.
* --allow-broken-links - show the sh_link of a symbol table is set to 0
  when its string table has been removed when this option is specified.
* --weaken-symbol(s) - show that symbols that are not mentioned are not
  weakened, if they would otherwise be, and that missing symbols from
  the list do not cause problems.
* --wildcard - show the wildcard behaviour for several options that were
  previously unchecked.

Reviewed by: alexshap

Differential Revision: https://reviews.llvm.org/D97666