context: test assumes empty env variables.
context: test assumes empty env variables.

- I noticed a failure when I was using a local instance of directory
  set via an environment variable. This fixes the problem.

Change-Id: I9121f912e920b6e3230c0e4c14ae41abdacedde5
Reviewed-on: https://upspin-review.git.corp.google.com/1581
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin