test: log the MongoDB URI on failure to connect

[blgm]

Technically this means logging a credential, but...

• It's a credential that's not functional
• It's a credential for an ephemeral database with nothing of value in it
• It's only visible to a user who can run cf logs and in practice all the users who can do that can run cf ssh and see the credential in /proc/*/environ (in principle you can have different permissions for this, but in practice this is not done on test environments)
• So failure to log the URI just makes debugging harder - it's not adding any real security