unittests shouldn't hardcode relative paths

[umlaeute]

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the "json-encode/read-large" unit-tests hardcodes a relative path to [json-encode]: https://github.com/residuum/PuRestJson/blob/b091c4b043015945a34f41c46ff16b6fc9a06219/unittests/json-encode/read-large.pd#L8

in order to be able to run the unit-tests on an already-installed package, this should be changed to just [json-encode] (and let Pd handle the finding of the "correct" binary).

you might also add some flags to the unittest-scripts to allow for specifying the search-paths