[libc++] Clean up test_iterators.h. NFCI.

The majority of the changes here are whitespace.
Also simplify `ThrowingIterator`'s bookkeeping (NFC).
Also move some free operators into hidden friends, for sanity's sake.
Also `=delete` some more comma operators.
Also use `constexpr` in C++20 instead of `TEST_CONSTEXPR_CXX14`.

Differential Revision: https://reviews.llvm.org/D103341