Reduce work level for stochastic tests.

The LevelDB stochastic tests could take a long time.

Original:
```
Matt:prometheus mtp$ make test
make -C model
protoc --go_out=generated/ data.proto
go build ./...
go test ./...
warning: building out-of-date packages:
	github.com/matttproud/golang_instrumentation/maths
	github.com/matttproud/golang_instrumentation/utility
	github.com/matttproud/golang_instrumentation/metrics
	github.com/matttproud/golang_instrumentation
installing these packages with 'go test -i ./...' will speed future tests.

?   	github.com/matttproud/prometheus	[no test files]
?   	github.com/matttproud/prometheus/coding	[no test files]
ok  	github.com/matttproud/prometheus/coding/indexable	0.012s
?   	github.com/matttproud/prometheus/model	[no test files]
?   	github.com/matttproud/prometheus/model/generated	[no test files]
?   	github.com/matttproud/prometheus/retrieval	[no test files]
?   	github.com/matttproud/prometheus/storage/metric	[no test files]
ok  	github.com/matttproud/prometheus/storage/metric/leveldb	70.800s
?   	github.com/matttproud/prometheus/storage/raw	[no test files]
?   	github.com/matttproud/prometheus/storage/raw/index	[no test files]
ok  	github.com/matttproud/prometheus/storage/raw/index/leveldb	0.012s
ok  	github.com/matttproud/prometheus/storage/raw/leveldb	0.016s
ok  	github.com/matttproud/prometheus/utility	0.012s
?   	github.com/matttproud/prometheus/utility/test	[no test files]
```

to

```
Matt:prometheus mtp$ make test
make -C model
protoc --go_out=generated/ data.proto
go build ./...
go test ./...
warning: building out-of-date packages:
	github.com/matttproud/golang_instrumentation/maths
	github.com/matttproud/golang_instrumentation/utility
	github.com/matttproud/golang_instrumentation/metrics
	github.com/matttproud/golang_instrumentation
installing these packages with 'go test -i ./...' will speed future tests.

?   	github.com/matttproud/prometheus	[no test files]
?   	github.com/matttproud/prometheus/coding	[no test files]
ok  	github.com/matttproud/prometheus/coding/indexable	0.011s
?   	github.com/matttproud/prometheus/model	[no test files]
?   	github.com/matttproud/prometheus/model/generated	[no test files]
?   	github.com/matttproud/prometheus/retrieval	[no test files]
?   	github.com/matttproud/prometheus/storage/metric	[no test files]
ok  	github.com/matttproud/prometheus/storage/metric/leveldb	2.158s
?   	github.com/matttproud/prometheus/storage/raw	[no test files]
?   	github.com/matttproud/prometheus/storage/raw/index	[no test files]
ok  	github.com/matttproud/prometheus/storage/raw/index/leveldb	0.013s
ok  	github.com/matttproud/prometheus/storage/raw/leveldb	0.013s
ok  	github.com/matttproud/prometheus/utility	0.013s
?   	github.com/matttproud/prometheus/utility/test	[no test files]
```