unit/mbyte_spec: Run utf_char2bytes test in batches of 0xFFF characters

Running the full 0xFFFF set of tests in one process fails on slower
architectures/when there's CPU contention.

ref https://github.com/neovim/neovim/issues/9001#issuecomment-421843790

Steps to reproduce:
    :set verbose=9
    :call system(['echo'])
    E730: using List as a String

libuv users are only supposed to directly include uv.h.  In v1.21.0, all
the uv-*.h headers were renamed to uv/*.h, which caused the unit tests
to fail with

    [123/125] Generating post/uv-errno.h
    FAILED: test/includes/post/uv-errno.h
    cd «SRCDIR»/src/neovim/build/test/includes && /usr/bin/clang -std=c99 -E -P «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h -I/usr/include -I/usr/include -o «SRCDIR»/neovim/build/test/includes/post/uv-errno.h
    «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h:1:10: error: 'uv-errno.h' file not found with <angled> include; use "quotes" instead
    #include <uv-errno.h>
             ^~~~~~~~~~~~
             "uv-errno.h"

The intention of the file is to extend libuv's error constants with more
values used by the unit tests.  This can just as easily be achieved
without poking into pseudo-private header files.

Improve coverage of `build_stl_str_hl`.
Minor removal of dead code in the tested function.

Use LuaJIT FFI to create char pointer.
Validate output with utf_ptr2char(), vim_iswordc() and vim_iswordp().
Use const for LuaJIT string-to-char conversion.