Rename keymgr constants to match file names

The following discussion from !2125 (merged) should be addressed:

• @Diziet started a discussion: (+1 comment)

  This sorting-out is an improvement, but we still have a rather odd situation where the Rust constant names and the test data filenames don't precisely correspond. And the source code text in this module is full of clumsy boilerplate.

  I wonder if we could macro this up some more. It might involve renaming the files to match the constant names (modulo case conversion by paste)