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/etcd/db_test.go

@@ -19,7 +19,7 @@ func TestDump(t *testing.T) {
 
 	f := NewEtcdTestFixture(t)
 
-	db, err := newEtcdBackend(context.TODO(), f.BackendConfig())
+	db, err := newEtcdBackend(context.Background(), f.BackendConfig())
 	require.NoError(t, err)
 
 	err = db.Update(func(tx walletdb.ReadWriteTx) error {