Introduce structure to the test/ directory

This introduces a clear separation between test data and test
binaries. Test data is moved into test/data, and the test binaries
move into test/src. A new CMake script specific to building the
tests is introduced in /test to slightly clean up the toplevel
one.

As well as tidying things up, this makes the next step trivial...

test/data/json.org/2.json

@@ -0,0 +1,11 @@
+{"menu": {
+  "id": "file",
+  "value": "File",
+  "popup": {
+    "menuitem": [
+      {"value": "New", "onclick": "CreateNewDoc()"},
+      {"value": "Open", "onclick": "OpenDoc()"},
+      {"value": "Close", "onclick": "CloseDoc()"}
+    ]
+  }
+}}