I am trying to do some UI testing in isolation on some fragments that uses a shared Bottom navigation view from MainActivity, basically used to navigate and to scroll up and down, but my tests fails with nullpointerException basically because the buttons inside the bottom navigation view are not found in the fragment layout. My question is how to pass these layouts belonging to mainactivity to the fragmentTest class.
FragmentScenario is the wrong approach, because a Fragment should not even depend on the parent Activity (and if it does, always check with instanceof or is, which one Activity it actually had been attached to). Better instrument the parent Activity with an ActivityTestRule instead, because a FragmentScenario uses it's own mock Activity and so you'll never get a handle to the expected one parent Activity (that's intentional, in order to rule out rigid dependencies to the parent Activity). Just set a break-point inside your current test's code, in order to see that there is no BottomNavigationView present, because it hasn't been inflated.
There's also a new ActivityScenario (which is currently still in beta stage).