[tests] new test to check clang's warning has been silenced

tests/syntax/clang_redef_warning.c

+/* run.config
+ENABLED_IF: %{bin-available:clang}
+OPT: -cpp-command="clang -C -E -I." -cpp-frama-c-compliant -print
+*/
+
+#include <stddef.h>
+
+void f(void) { }

tests/syntax/oracle/clang_redef_warning.res.oracle

+[kernel] Parsing clang_redef_warning.c (with preprocessing)
+/* Generated by Frama-C */
+void f(void)
+{
+  return;
+}
+
+