Instead of copying the test executable and the JSON files used by
the tests at install time, define CMake/CTest tests for running
the json_unit executable from any build directory with the project's
source directory as its working directory.

- call enable_testing in the main lists file to allow the definition
of tests
- remove install commands from the test directory's lists file
- define two tests
  - json_unit_default for running the default tests by executing
json_unit without any arguments
  - json_unit_all for running all the tests by executing json_unit
with the "*" argument
- update the AppVeyor configuration file to use the new testing method