tests: Add json testrunner

* configure.ac: Configure makefile.
* tests/Makefile.am: Run json tests if gpg tests are run.
* tests/json/t-json.c: New testrunner for json tests.
* tests/json/t-config.in, tests/json/t-config.out: First test.

The idea of this test runner is that it only looks for parts
in the output. This should allow it to write robust tests
that check for the basics in the output but don't fail when
the output is extended or slightly changed.