Fix up vote tests in test_dir_handle_get.

These tests used a hardcoded vote with a hardcoded digest.  That
vote didn't have any pr lines, and so it is now invalid. I've
adjusted the testing code so that it mocks the signature-checking,
so that we can more easily change the contents of the vote.

src/feature/dirparse/sigcommon.c

@@ -139,13 +139,13 @@ signed_digest_equals, (const uint8_t *d1, const uint8_t *d2, size_t len))
  * the document when generating log messages.  Return 0 on success, negative
  * on failure.
  */
-int
-check_signature_token(const char *digest,
+MOCK_IMPL(int,
+check_signature_token,(const char *digest,
                       ssize_t digest_len,
                       directory_token_t *tok,
                       crypto_pk_t *pkey,
                       int flags,
-                      const char *doctype)
+                      const char *doctype))
 {
   char *signed_digest;
   size_t keysize;

src/feature/dirparse/sigcommon.h

@@ -20,12 +20,12 @@ int router_get_hash_impl(const char *s, size_t s_len, char *digest,
 
 #define CST_NO_CHECK_OBJTYPE  (1<<0)
 struct directory_token_t;
-int check_signature_token(const char *digest,
-                          ssize_t digest_len,
-                          struct directory_token_t *tok,
-                          crypto_pk_t *pkey,
-                          int flags,
-                          const char *doctype);
+MOCK_DECL(int, check_signature_token,(const char *digest,
+                                     ssize_t digest_len,
+                                     struct directory_token_t *tok,
+                                     crypto_pk_t *pkey,
+                                     int flags,
+                                     const char *doctype));
 
 int router_get_hash_impl_helper(const char *s, size_t s_len,
                             const char *start_str,