Fix issues with experimental_mixed_language_library header map

[luispadron]

Fixes some issues I ran into while integrating on a more real project than the examples:

• includes is a frozen list after kwargs.pop, use [] + kwargs.pop to allow mutation within the macro.
• objc_copts and append do not work when objc_copts is a select. Use += to add a default list of copts to any list or select
• Add testonly support to header_map_support