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.

Previously, we ignored only _Float128. But glibc 2.27 added _Float32
and _Float32x.  Rather than play whack-a-mole, ignore everything.