Added missing test files.

[simonpf]

Fixes #376.

[olemke]

The current test failures in the Github Actions are caused by a broken Ubuntu mirror server and are not related to what I mention below. Fixed

Which version of keras are you using? The tests work for me on macOS with keras 2.3.1, but on Linux with keras 2.4.3 I get this error for test_save_qrnn:

___________________________________ TestQrnn.test_save_qrnn[keras] ____________________________________

self = <typhon.tests.retrieval.qrnn.test_qrnn.TestQrnn object at 0x7f1a80038790>, backend = 'keras'

    @pytest.mark.parametrize("backend", backends)
    def test_save_qrnn(self, backend):
        """
        Test saving and loading of QRNNs.
        """
        set_backend(backend)
        qrnn = QRNN(self.x_train.shape[1], np.linspace(0.05, 0.95, 10))
        f = tempfile.NamedTemporaryFile()
>       qrnn.save(f.name)

../../../.local/lib/python3.7/site-packages/typhon/tests/retrieval/qrnn/test_qrnn.py:91:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../../.local/lib/python3.7/site-packages/typhon/retrieval/qrnn/qrnn.py:625: in save
    backend.save_model(f, self.model)
../../../.local/lib/python3.7/site-packages/typhon/retrieval/qrnn/models/keras.py:26: in save_model
    keras.models.save_model(model, f)
../../../miniconda3/lib/python3.7/site-packages/tensorflow/python/keras/saving/save.py:123: in save_model
    os.path.splitext(filepath)[1] in _HDF5_EXTENSIONS):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

p = <_io.BufferedWriter name='/tmp/tmpmhh5x6b8'>

    def splitext(p):
>       p = os.fspath(p)
E       TypeError: expected str, bytes or os.PathLike object, not _io.BufferedWriter

../../../miniconda3/lib/python3.7/posixpath.py:122: TypeError