Organize test-cases into folders, extract compatibility test into proper test, support adding test-case specific tests

betterproto/tests/inputs/int32/int32.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+// Some documentation about the Test message.
+message Test {
+    // Some documentation about the count.
+    int32 count = 1;
+}