test: use read-only account

Don't write in plaintext the password of an account which has full
control over the repository. Instead use an account with read-only
access.
test: remove assembla clone test

The assembla failure we were seeing referred to a private repository,
which is not what is there at the moment.

This reverts 1fd21b0342f
Merge pull request #2425 from libgit2/cmn/clone-auth-change

Don't use read-write accounts for the tests