Refactor the "check categories and severity" logic in diag.t

Refactor the "check categories and severity" logic in diag.t

Move the "todo" bailout as early as possible, before any work is done.
Replace C<use 5.01> [sic] with C<use feature 'state'>, as this makes the
intent much clearer.
Use parenthesis for like() and is() as this is consistent with the rest of the
test, and permits a simplification of the BEGIN handling.