added examples for tests that compare the compiler output with a reference

test/ref/custom-reference.c

@@ -0,0 +1,24 @@
+
+/*
+    this is an example (not actually a regression test) that shows how to
+    make a check that compares the compiler (error-) output with a provided
+    reference.
+
+    to produce a reference file, first make sure your program "works" as intended,
+    then "make" in this directory once and copy the produced compiler output to
+    the reference:
+
+    $ cp ../../testwrk/ref/custom-reference.g.6502.out custom-reference.cref
+
+    and then "make" again to confirm
+*/
+
+#include <stdint.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[])
+{
+    printf("%02x", 0x42);
+    /* produce a warning */
+    return 0;
+}