Until C++20, `std::find_if(_not)` were not `constexpr`, so uses of
`TEST_CONSTEXPR_CXX17` make the test functions ill-formed, no diagnostic
required in C++17 mode.
The uses were introduced in 2b4b26ea84fd9c95d0ff25ce338c15ea5e74a4e4,
while the transformation looked incorrect - `TEST_CONSTEXPR_CXX20`
should be used instead.
Drive-by changes: Fix comments in both changed test files to match the
actually standardized wording.
11c0fd4f44