Skip unit/d8/8-performance in debug mode

This test often fails in debug mode because it is too slow. The error
would look something like:

```
Timer difference too big: 1.8240000000000691ms
Timer difference too big: 1.893000000000029ms
Timer difference too big: 1.8870000000000573ms
Timer difference too big: 1.8909999999999627ms
Timer difference too big: 1.8869999999999436ms
Timer difference too big: 1.8960000000000719ms
Timer difference too big: 1.884999999999991ms
test/mjsunit/mjsunit.js:316: Failure: expected <true> found <false>
```