Tests of market efficiency are the joint tests of (1)the market is efficient and (2) expected return model.
Please help me (a) explain this and (b) is it possible to test market efficiency with either of these ones?
When markets are said to be efficient then the expectation is that there is no excess returns (alpha).
The expected return is basically a model of forecasting returns such as CAPM.
So basically you want to test if alpha (excess returns) is significant.
It's a joint test in the sense that you are not only testing alpha but are also testing the validity of CAPM to determine returns in the first instance.