kvdb/etcd: remove context.TODO() from test helpers

We want `context.TODO()` to be high signal in the code-base. It should
signal clearly that work is required to thread parent context through to
the call-site. So to keep the signal-to-noise ratio high, we remove any
context.TODO() calls from tests since these will never need to be
replace by a parent context.

After this commit, there is only a single context.TODO() left in the
code-base.

kvdb/backend.go

@@ -274,7 +274,7 @@ func GetTestBackend(path, name string) (Backend, func(), error) {
 			return nil, empty, err
 		}
 		backend, err := Open(
-			EtcdBackendName, context.TODO(), etcdConfig,
+			EtcdBackendName, context.Background(), etcdConfig,
 		)
 		return backend, cancel, err