tmpfs workaround

[Aram Hăvărneanu]

> For reasons we're not quite sure of, mongo is creating ~ 250mb of scratch space during each test run.

I think the reason is well understood. MongoDB creates a 200MB capped
collection and it allocates contiguous space for that at startup. For
the state tests we override that 200MB to 1MB in an export_test.go,
but that obviously doesn't do anything for other packages that import
state.