Minor refactoring of Dashtap.
Sketching source dependency tests.

No actual code change, just sketching of ideas.

* Regularized test filenames a bit.

* Added a bunch of source dependency related tests to the list of tests
  to implement.
Added some more document comments to dashtap.
Added 'match' function to Dashtap + tests.
Fixed bug in Dashtap test for 'error'.
Renamed test framework tests to 'dashtap-...'.
Ooops! Bug in 'match' test fixed.
Added SKIP function to Dashtap.

* Added SKIP/END_SKIP functions.

* Added tests for SKIP/END_SKIP and TODO/END_TODO.
Dashtap 'seteval' now checks VARNAME arg.

* VARNAME argument to function 'seteval' is checked to see that it is a
  valid variable name before running.
Dashtap 'execute' now uses one less 'trap'.

* Unnecessarily invoked 'trap' twice.
Optimized Dashtap 'descr' function.

* Replaced one occurance of $(...) with eval.