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/5.json

@@ -0,0 +1,27 @@
+{"menu": {
+    "header": "SVG Viewer",
+    "items": [
+        {"id": "Open"},
+        {"id": "OpenNew", "label": "Open New"},
+        null,
+        {"id": "ZoomIn", "label": "Zoom In"},
+        {"id": "ZoomOut", "label": "Zoom Out"},
+        {"id": "OriginalView", "label": "Original View"},
+        null,
+        {"id": "Quality"},
+        {"id": "Pause"},
+        {"id": "Mute"},
+        null,
+        {"id": "Find", "label": "Find..."},
+        {"id": "FindAgain", "label": "Find Again"},
+        {"id": "Copy"},
+        {"id": "CopyAgain", "label": "Copy Again"},
+        {"id": "CopySVG", "label": "Copy SVG"},
+        {"id": "ViewSVG", "label": "View SVG"},
+        {"id": "ViewSource", "label": "View Source"},
+        {"id": "SaveAs", "label": "Save As"},
+        null,
+        {"id": "Help"},
+        {"id": "About", "label": "About Adobe CVG Viewer..."}
+    ]
+}}