Evaluate JEXL subexpressions in desktop integration tests

[jaredlockhart]

As a followup to https://github.com/mozilla/experimenter/issues/7449 we discovered that while we can evaluate subexpressions in the Rust JEXL evaluator and meaningfully determine if they include undefined variables or invalid comparisons, we can't do the same on desktop without changes to the JS JEXL evaluator.

After we make the following changes:

https://github.com/mozilla/mozjexl/issues/25

And get them landed into MC:

https://bugzilla.mozilla.org/show_bug.cgi?id=1773388#c2

then we can add the subexpression evaluation to desktop the way we do for the sdk.

┆Issue is synchronized with this Jira Task