* Update scan.py to be more flexible with output files

  Previously, scan.py insisted that tests were separated into "C++" and "other"
  tests. While fine for ReForm, this does not fit the gtest model we want to use
  in ReForm Preprocess. scan.py can now be configured to take any number of
  categories of tests, with no restriction on extensions.

* Add ability for scan.py to use a format

  Rather than taking the argument 'command' and prescribing the format

    command(<rel_path>)

  scan.py now takes a 'format' argument, in which instances of {relative_path}
  and {safe_relative_path} are respectively replaced with the relative path and
  a 'safe' version of the relative path (i.e. non-alphanumeric characters
  replaced by _). The above format would be written

    command({relative_path})

  This allows for much greater flexibility in the generated files.