Added some error checking for Dashtap functions.

* All Dashtap's test functions now check the number of arguments.
Added Dashtap function 'isnt' + tests for it.
Added Dashtap function 'reset_timestamp'.

* Plus the helper functions 'timestamp_file' and 'timestamp_time'.
Modified the workings of 'init_test' + added test.

* 'init_test' no longer (implicitly) populate the test tempdir with
  subdirectories, instead this have to be done manually (using 'mkdir'). This
  clarifies the test scripts, since the directory creation now is explicity.

* Added helper function 'cpdir' which copies a dir from a test scripts data dir
  to the tempdir where it's executed.

* Also added test script '19-rebuild-target-modified-but-time-same.t' (which
  needed the above changes in order to work), which tests that fix look only at
  the contents (not the file mtime) when determining whether a target file has
  been modified since last build.
Minor rephrasing of descriptions in test scripts.