[tests] avoid having an absolute path in an oracle

tests/syntax/anon_enum_libc.c

 /* run.config
+FILTER: sed -e 's|#include *"\([^/]*[/]\)*\([^/]*\)"|#include "PTESTS_DIR/\2"|'
 OPT: -cpp-extra-args="-I @PTEST_DIR@" -ocode @PTEST_DIR@/result/@PTEST_NAME@.c -print -then -ocode="" @PTEST_DIR@/result/@PTEST_NAME@.c -print
 */
 

tests/syntax/oracle/anon_enum_libc.res.oracle

 [kernel] Parsing tests/syntax/anon_enum_libc.c (with preprocessing)
 [kernel] Parsing tests/syntax/result/anon_enum_libc.c (with preprocessing)
 /* Generated by Frama-C */
-#include "/home/virgile/Frama-C/frama-c-stable/tests/syntax/anon_enum_libc.h"
+#include "PTESTS_DIR/anon_enum_libc.h"
 struct __anonstruct_s1_1 {
    int x ;
    float y ;