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/src/fuzz.cpp

@@ -0,0 +1,34 @@
+/*
+    __ _____ _____ _____
+ __|  |   __|     |   | |  JSON for Modern C++ (fuzz test support)
+|  |  |__   |  |  | | | |  version 2.0.0
+|_____|_____|_____|_|___|  https://github.com/nlohmann/json
+
+Run "make fuzz_testing" and follow the instructions.
+
+Licensed under the MIT License <http://opensource.org/licenses/MIT>.
+*/
+
+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+#ifdef __AFL_HAVE_MANUAL_CONTROL
+    while (__AFL_LOOP(1000))
+    {
+#endif
+        try
+        {
+            json j(std::cin);
+            std::cout << j << std::endl;
+        }
+        catch (std::invalid_argument& e)
+        {
+            std::cout << "Invalid argument in parsing" << e.what() << '\n';
+        }
+#ifdef __AFL_HAVE_MANUAL_CONTROL
+    }
+#endif
+}