Defer GC and scrub ivars to speed up test suite

On my poorly old MBP:

Test suite without GC tweaks: 22m45
Test suite with GC tweaks:    19m38

So by my unscientific calculations, test suite run time is improved
by 13.7%. In theory, this should also help with memory usage, although
I have no measurements for this.
Merge pull request #258 from alphagov/defer-gc

Some tweaks to speed up the test suite.  This has no related story in pivotal.