Defer garbage collection for 10s to speed up tests

**DO NOT MERGE**. Testing this in Jenkins.

**Before**

```
Finished tests in 104.873295s, 1.3349 tests/s, 4.3672 assertions/s.
```

**After**

```
Finished tests in 89.776439s, 1.5594 tests/s, 5.1016 assertions/s.
```