Start moving the tests into subfolders. Transition to SCons for build system for the tests.

Only a few tests updated so far. Have to include all the rest before merging to mainline.

Update issue 63
Status: Started

tests/multiple_files/callbacks2.proto

@@ -0,0 +1,9 @@
+// Test if including generated header file for this file + implicit include of
+// callbacks.pb.h still compiles. Used with test_compiles.c.
+import "callbacks.proto";
+
+message Callback2Message {
+    required TestMessage tstmsg = 1;
+    required SubMessage submsg = 2;
+}
+