[tests] add test case for builtin macro censorship

tests/misc/custom_machdep.c

 /* run.config*
- OPT: -machdep %{dep:@PTEST_DIR@/@PTEST_NAME@.yaml} -print
+ MACRO: machdep %{dep:@PTEST_DIR@/@PTEST_NAME@.yaml}
+ OPT: -machdep @machdep@ -print
+ COMMENT: we can't only use -D, as the __fc_machdep.h define takes precedence
+ COMMENT: with -U, our cmdline definition is used in the code
+ OPT: -machdep @machdep@ -cpp-extra-args="-UCUSTOM_MACHDEP -DCUSTOM_MACHDEP=42" -print
 */
 // most of the following includes are not directly used, but they test if
 // the custom machdep has defined the necessary constants

tests/misc/oracle/custom_machdep.1.res.oracle

+[kernel] Parsing custom_machdep.c (with preprocessing)
+/* Generated by Frama-C */
+#include "ctype.h"
+#include "errno.h"
+#include "inttypes.h"
+#include "locale.h"
+#include "math.h"
+#include "signal.h"
+#include "stdarg.h"
+#include "stddef.h"
+#include "stdint.h"
+#include "stdio.h"
+#include "stdlib.h"
+#include "string.h"
+#include "strings.h"
+#include "time.h"
+#include "wchar.h"
+int main(void)
+{
+  int __retres;
+  __retres = 8388607 - 42;
+  return __retres;
+}
+
+