In 9d999d7 waiting for sessionContext.ready was removed, this
caused _loadNotebook() not to work, because it was called before the
widget metadata was available.

This commit restores waiting for sessionContext.ready. This has
to be before _loadFromKernel(), otherwise this.kernel is not
present, causing an exeption, skipping _loadNotebook().

Fixes: jupyterlab/jupyterlab#15361

See jupyter-widgets#3883

This will make the galata report all the mismatches at the end of the
test run, instead of failing immediately on the first mismatch.

Unsure why this is needed, since jupyter-widgets#3866 seems like an unrelated change.