[fenix] Close https://github.com/mozilla-mobile/fenix/issues/19459: Use Store.waitUntilIdle in intermittent failing test (https://github.com/mozilla-mobile/fenix/pull/19683)

app/src/test/java/org/mozilla/fenix/nimbus/NimbusBranchesControllerTest.kt

@@ -7,7 +7,8 @@ package org.mozilla.fenix.nimbus
 import io.mockk.mockk
 import io.mockk.verify
 import mozilla.components.service.nimbus.NimbusApi
-import mozilla.components.support.test.mock
+import mozilla.components.support.test.libstate.ext.waitUntilIdle
+import org.junit.Assert.assertEquals
 import org.junit.Before
 import org.junit.Test
 import org.mozilla.experiments.nimbus.Branch
@@ -23,7 +24,7 @@ class NimbusBranchesControllerTest {
 
     @Before
     fun setup() {
-        nimbusBranchesStore = mock()
+        nimbusBranchesStore = NimbusBranchesStore(NimbusBranchesState(emptyList()))
         controller = NimbusBranchesController(nimbusBranchesStore, experiments, experimentId)
     }
 
@@ -36,9 +37,12 @@ class NimbusBranchesControllerTest {
 
         controller.onBranchItemClicked(branch)
 
+        nimbusBranchesStore.waitUntilIdle()
+
         verify {
             experiments.optInWithBranch(experimentId, branch.slug)
-            nimbusBranchesStore.dispatch(NimbusBranchesAction.UpdateSelectedBranch(branch.slug))
         }
+
+        assertEquals(branch.slug, nimbusBranchesStore.state.selectedBranch)
     }
 }