Refactor Tests

[vdutor]

The unit and integration tests are growing, and we are reusing a lot of the same code, in particular to:

• generate data
• run training
• compile models

We should have some common functions and fixtures which will both increase readability of code, reduce duplication and make most of pytest caching.

As part of this, could we introduce an assert_shapes() utility function instead of the kinda confusing assert np.all(tf.shape(tensor) == tensor_shape)?

[st--]

Minor clean-up that can happen alongside: np.testing.assert_almost_equal, np.testing.assert_array_almost_equal and np.testing.assert_approx_equal are used in gpflux but deprecated by numpy. We should move to np.testing.assert_allclose or one of the ULP tests.

[st--]

Also, we might want to consider moving shape comments into code-asserts of tensor shapes within the actual code.