Tests: re-organize tests, comments.

This commit merely moves some tests around and corrects/extends some comments.
Tests: more re-organization.

More test re-organization:
- split the FLAC tests from the rest to make spotting missing FLAC support in
  libsndfile easier
- split up some of the tests
- remove redundant tests from test cases
- add/improve comments
Make some small stylistic changes.
Use construction variable substitution more.

Throughout the build system I was concatenating variables by doing
"env[some_var] + env[another_var]".  It turns out SCons lets you reference the
variables in strings directly using variable substitution (e.g.,
"env.File('some_string'+'${some_var}')").  This makes some things easier to
read.