[tests] avoid race condition in test

When tests are run in parallel, using the same name for two test outputs might
truncate it before it is written, leading to a non-deterministic oracle.

tests/syntax/empty_union.i

 /* run.config*
-   STDOPT: +"-machdep gcc_x86_32 -print -ocode @PTEST_NAME@_reparse.c -then @PTEST_NAME@_reparse.c -ocode=''"
-   STDOPT: +"-machdep msvc_x86_64 -print -ocode @PTEST_NAME@_reparse.c -then @PTEST_NAME@_reparse.c -ocode=''"
+   STDOPT: +"-machdep gcc_x86_32 -print -ocode @PTEST_NAME@_@PTEST_NUMBER@_reparse.c -then @PTEST_NAME@_@PTEST_NUMBER@_reparse.c -ocode=''"
+   STDOPT: +"-machdep msvc_x86_64 -print -ocode @PTEST_NAME@_@PTEST_NUMBER@_reparse.c -then @PTEST_NAME@_@PTEST_NUMBER@_reparse.c -ocode=''"
  EXIT: 1
    STDOPT:
  */

tests/syntax/oracle/empty_union.0.res.oracle

 [kernel] Parsing empty_union.i (no preprocessing)
-[kernel] Parsing empty_union_reparse.c (with preprocessing)
+[kernel] Parsing empty_union_0_reparse.c (with preprocessing)
 /* Generated by Frama-C */
 union empty {
    

tests/syntax/oracle/empty_union.1.res.oracle

 [kernel] Parsing empty_union.i (no preprocessing)
-[kernel] Parsing empty_union_reparse.c (with preprocessing)
+[kernel] Parsing empty_union_1_reparse.c (with preprocessing)
 /* Generated by Frama-C */
 union empty {