[tests] Add a test for statement attributes

tests/syntax/oracle/stmt_attributes.0.res.oracle

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

tests/syntax/oracle/stmt_attributes.1.res.oracle

+[kernel] Parsing stmt_attributes.c (with preprocessing)
+[kernel] stmt_attributes.c:17: 
+  syntax error:
+  Location: line 17, column 30, before or at token: ;
+  15        attributes specifiers, which is forbidden, so a syntax error is raised.
+  16        Statement attributes are not supported in frama-c except for labels. */
+  17      __attribute__((fallthrough));
+                                      ^
+  18      #endif
+  19    }
+[kernel] Frama-C aborted: invalid user input.

tests/syntax/stmt_attributes.c

+/* run.config
+  STDOPT:
+  EXIT: 1
+  STDOPT: #"-cpp-extra-args=-DNO_SPECIFIERS"
+*/
+
+void f() {
+
+  /* Label attributes are accepted in Frama-C's parser, but dumped when building
+     Cabs. */
+  foo: __attribute__((unused));
+
+  #ifdef NO_SPECIFIERS
+  /* Parser detect this as a declaration without any specifiers except for GCC's
+    attributes specifiers, which is forbidden, so a syntax error is raised.
+    Statement attributes are not supported in frama-c except for labels. */
+  __attribute__((fallthrough));
+  #endif
+}