test: fix missing type in error.E
test: fix missing type in error.E

Change-Id: I2e3a5558009594378d733a679778c010b5c48c7a
Reviewed-on: https://upspin-review.googlesource.com/5500
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin