Rearrange default_configuration and CheckCfg::fill_well_known.

There are comments saying these two functions should be kept in sync,
but they have very different structures, process symbols in different
orders, and there are some inconsistencies.

This commit reorders them so they're both mostly processing symbols in
alphabetical order, which makes cross-checking them a lot easier. The
commit also adds some macros to factor out repetitive code patterns.

The commit also moves the handling of `sym::test` out of
`build_configuration` into `default_configuration`, where all the other
symbols are handled.