Renamed Fix.sh tests + small test cleanup.

* Instead of prefixing each test with a number (e.g. '13') they're now
  prefixed with a letter + number (e.g. 'b08'), where the letter denotes
  what kind of test it is (as specified by in the org-mode TESTS.txt).
  This will lead to fewer test having to be renamed to keep the sequence
  ordered in the future.

* Also fixed some small issues here and there.
Added source dependency tests (as TODO tests).

* Also modified Dashtap functions 'file_is', 'reset_timestamp',
  'is_unchanged' and 'is_changed' so that they no fail gracefully if the
  file they're testing doesn't exist. (This was needed in order to make
  several of the TODO tests work properly.)
Improved Dashtap 'end_*' functions + SKIP mode.

* 'END_TODO', 'END_SKIP' and 'end_title' now all complain if called
  without the corresponding mode/title first having been set.

* 'pass' and 'fail' now adheres to SKIP mode, even if they're called
  directly. (Previously SKIP mode was only detected by the test
  functions, but this doesn't work if a user uses 'pass'/'fail' directly
  in their test scripts.)