Use fixed secret in AuthManager tests

Not very important for current code, as AuthManager is the only
reader/writer of access tokens. For macaroons we will need to make
sure that representation is stable to avoid making tokens incompatible
with external code.

BUG=25934385

Change-Id: I3f31814937b4b2806dd32387c0b5cb8deb53b30b
Reviewed-on: https://weave-review.googlesource.com/1860
Reviewed-by: Alex Vakulenko <avakulenko@google.com>