Report disabled tests

[saper]

Directories which do not have valid input files (or output files before #494) are silently skipped.
This is how not-really-magic disabled keyword works (renaming input.scss to input.disabled.scss).

> ruby sass-spec.rb -c ~/sw/sass/bin/sass -s spec/libsass-todo-tests/css_error_with_windows_newlines
Recursively searching under directory 'spec/libsass-todo-tests/css_error_with_windows_newlines' for test files to test '/home/saper/sw/sass/bin/sass' with.
Sass 3.4.14.2e0f33b (Selective Steve)
Run options: --seed 19577

# Running:



Finished in 0.001196s, 0.0000 runs/s, 0.0000 assertions/s.

0 runs, 0 assertions, 0 failures, 0 errors, 0 skips

This is confusing especially given there is always a chance for a typo. See also #501 and #505 for funny cases where it confuses people.

I think we should add those directories to "skips" to indicate this. Especially useful when running against a single or a small group of directories.

[saper]

Of course, directories containing only some other directories (sub-tests) should not be reported.

[chriseppstein]

There's no disabled concept any more. tests can be marked as todo.