Remove mock usage from test_client

[exarkun]

https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3520

[sajith]

The changes in test look equivalent to the prior revision to me.

Incidentally, I have been toying with GitHub's security scanning tools on my fork of Tahoe-LAFS. CodeQL generated three warnings:

https://github.com/sajith/tahoe-lafs/security/code-scanning?query=tool%3ACodeQL

Two of the warnings are about the use of mktemp(), but since that is in the test suite, I'm not sure we should worry. The other one is about creation of 1024-bit RSA keys, also possibly from the test suite, so I'm not sure those are worth worrying about too.

tahoe-lafs/src/allmydata/test/test_crypto.py

Line 442 in 0e4cf0d

priv, pub = rsa.create_signing_keypair(1024)

tahoe-lafs/src/allmydata/test/test_crypto.py

Line 454 in 0e4cf0d

priv, pub = rsa.create_signing_keypair(1024)

Just wanted to bring it to your attention because there's an mktemp() and an RSA key in this PR. :-)