This partially reverts the changes of an earlier commit, which made
some things asynchronous. The `NimbusFeatures.getAllVariables()` API
is not, in fact, asynchronous, so opening the internals is not
strictly necessary. This simplifies the expression and testing for
later.
N.b.: the existing implementation did not correctly test
`_openFirstRunPage`: that function was async and there was no `await`.
Therefore existing tests were asserting things about the function
definitions and not the function behaviours.
Differential Revision: https://phabricator.services.mozilla.com/D235886
b6d918b470