Create new sub directories for tests related to backtraces and for tests
related to `$this` being reserved in different places and not being usable or
reassignable.
Work towards GH-15631
While reviewing the existing tests in the `constexpr` directory, I found that
some of the names were not updated to reflect the contents when the contents
were changed in #9301.
Follow-up to #15638
Add support for new expressions inside parameter default values,
static variable initializers, global constant initializers and
attribute arguments.
RFC: https://wiki.php.net/rfc/new_in_initializers
Closes GH-7153.
