Fix crash-test for assertion failure

When we made assertions not get compiled in for the coverage case, we
missed one case where, for our tests, we really DO want to have an
assertion fail: the backtrace test.

Bugfix on 1228dd293b60a8eaab03472fa29428c5e2752c44; bug not in any
released tor

src/test/test_bt_cl.c

@@ -5,6 +5,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+/* To prevent 'assert' from going away. */
+#undef TOR_COVERAGE
 #include "or.h"
 #include "util.h"
 #include "backtrace.h"