test_relayselector.py hardcodes `/tmp` which breaks test_get_exits()

test/test_relayselector.py::TestRelaySelector::test_get_exits 
----------------------------------- live log call -----------------------------------
DEBUG    relayselector:relayselector.py:176 Selecting exits depending on parameters.
CRITICAL relayselector:relayselector.py:128 File "/tmp/cached-consensus" could not be read: [Errno 2] No such file or directory: '/tmp/cached-consensus'

I guess we should make use of test/data/cached-consensus as we do in other tests instead.