This makes it possible to refer to entities defined in the source file,
using the source names prefixed with $.
For example, $v20 refers to the value by that name in the sources, even
if it was renumbered to 'vx0' in the parsed file.
Create a new directory hierarchy under 'filetests' for all the tests
that are run by 'cton-util test'.
Convert the parser tests under 'tests/parser' to use 'test cat' and
filecheck directives.
