[libc++][format] Addresses LWG3839.

[libc++][format] Addresses LWG3839.

  LWG3839 range_formatter's set_separator, set_brackets, and
  underlying functions should be noexcept

Adds tests for:

  template<ranges::input_range R, class charT>
     struct range-default-formatter<range_format::sequence, R, charT>

These were missing, the format functions tests for the sequences
are already present.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D144286