Add a directory for tests that exercise the supported C standard differences.

As a first test added the snippet given in issue #1670

test/readme.txt

@@ -12,6 +12,13 @@ compiler is working as expected (when the tests behave as described):
 /val  - The bulk of tests are contained here, individual tests should exit with
         an exit code of EXIT_SUCCESS when they pass, or EXIT_FAILURE on error.
 
+/standard - like the tests in /val, the tests must exit with EXIT_SUCCESS on
+        success. Unlike the tests in /val these are not compiled for every
+        combination of optimizer options, but instead always with -Osir and then
+        for each supported C-standard (C89, C99, CC65). The goal is to use these
+        to check for regressions in standard conformance of the compiler and the
+        library.
+
 /ref  - These tests produce output that must be compared with reference output.
 
 /err  - contains tests that MUST NOT compile