[VM] Mark bigint test slow after small Smi change

[email protected]

Change-Id: I553c364be1ce2a29306f44f3a641441cdcb911ca
Reviewed-on: https://dart-review.googlesource.com/50223
Reviewed-by: Martin Kustermann <[email protected]>

Author: ErikCorryGoogle

tests/corelib_2/corelib_2.status

@@ -633,6 +633,6 @@ regexp/UC16_test: RuntimeError
 
 [ $hot_reload || $hot_reload_rollback ]
 bigint_parse_radix_test: Pass, Timeout # Issue 31659
-bigint_test: Pass, Crash # Issue 31660
+bigint_test: Pass, Slow, Crash # Issue 31660
 integer_parsed_mul_div_vm_test: Pass, Slow # Slow