Fix slownightly failure

getmore_error.js sets a failpoint to make the next OP_GETMORE fail, then
asserts that it does indeed fail. This caused a test failure in
small_oplog_passthough because the "next" OP_GETMORE came from
replication rather than the test. Moving the test into slowNightly which
should ensure that nothing else is running concurrently with it.
SERVER-10104 make mr_drop.js more likely to work