Add rudimentary unit testing via AIN file

Building 'test/test.pje' with the SDK compiler produces a file at
'test/Run/test.ain' which can be run with xsystem4 to test various
features.

Not the greatest testing method, but it works.

test/Source/System/System42/README.md

@@ -0,0 +1,12 @@
+Note
+----
+
+I am not 100% sure about the legality of distributing System40.exe and
+Sys42VM.dll here, so I've left them out. They can be found in the System 4 SDK,
+which you'll want to have anyways to compile these tests.
+
+Once you've copied the files, this directory should contain the following:
+
+    System40.exe
+    System40.ini
+    DLL\Sys42VM.dll