Random FunctionalJavascript failures: createCondition() does not wait for the edit form

Problem/Motivation

ConditionalFieldTestBase::createCondition() submits the
Add dependency form, which redirects to the
ConditionalFieldEditForm. Callers then immediately submit
Save settings or navigate to the listing page without waiting for
the redirect to finish.

This causes random FunctionalJavascript failures in different tests, usually
because the Save settings button or the newly created condition
cannot be found. The failures reproduce on an unmodified checkout, so this is a
test-infrastructure race rather than a module bug.

Proposed resolution

Add
$this->assertSession()->waitForButton('Save settings'); to
createCondition() so callers only continue once the edit form is
ready. One change in the base class covers all callers.

Remaining tasks

Verify the affected FunctionalJavascript tests run consistently without
intermittent failures.