Use mock_ptr_type() to silence compiler warnings

Unit tests on 32-bit architectures were failing due to
"cast to pointer from integer of different size" warnings.
mock_ptr_type silences those by first casting to uintptr_t.

tests/unittests/otr/stub_otr.c

@@ -31,12 +31,12 @@ void otr_shutdown(void) {}
 
 char* otr_libotr_version(void)
 {
-    return (char*)mock();
+    return mock_ptr_type(char*);
 }
 
 char* otr_start_query(void)
 {
-    return (char*)mock();
+    return mock_ptr_type(char*);
 }
 
 void otr_poll(void) {}
@@ -86,13 +86,13 @@ void otr_end_session(const char * const recipient) {}
 
 char * otr_get_my_fingerprint(void)
 {
-    return (char *)mock();
+    return mock_ptr_type(char *);
 }
 
 char * otr_get_their_fingerprint(const char * const recipient)
 {
     check_expected(recipient);
-    return (char *)mock();
+    return mock_ptr_type(char *);
 }
 
 char * otr_encrypt_message(const char * const to, const char * const message)