More realistic, failing BackstackTransitionsTest

`BackstackTransitionsTest` now uses a map, to expose problems hidden when key and content are the same thing. As a result we crash when popping.

Reproduces #63.

Author: rjrjr

compose-backstack/src/androidTest/java/com/zachklipp/compose/backstack/BackstackTransitionsTest.kt

@@ -43,6 +43,11 @@ class BackstackTransitionsTest {
     assertInitialStateWithMultipleScreens(Crossfade)
   }
 
+  @Test
+  fun transition_noop_forward() {
+    assertTransition(null, forward = true)
+  }
+
   @Test
   fun transition_slide_forward() {
     assertTransition(Slide, forward = true)
@@ -53,6 +58,11 @@ class BackstackTransitionsTest {
     assertTransition(Crossfade, forward = true)
   }
 
+  @Test
+  fun transition_noop_backward() {
+    assertTransition(null, forward = false)
+  }
+
   @Test
   fun transition_slide_backward() {
     assertTransition(Slide, forward = false)
@@ -82,19 +92,29 @@ class BackstackTransitionsTest {
     compose.onNodeWithText("one").assertDoesNotExist()
   }
 
-  private fun assertTransition(transition: BackstackTransition, forward: Boolean) {
-    val firstBackstack = listOf("one")
-    val secondBackstack = listOf("one", "two")
+  private fun assertTransition(transition: BackstackTransition?, forward: Boolean) {
+    val firstBackstack = mapOf(1 to "one")
+    val secondBackstack = mapOf(1 to "one", 2 to "two")
     var backstack by mutableStateOf(if (forward) firstBackstack else secondBackstack)
     compose.mainClock.autoAdvance = false
-    compose.setContent {
-      Backstack(
-        backstack,
-        frameController = rememberTransitionController(
-          animationSpec = animation,
-          transition = transition
-        )
-      ) { BasicText(it) }
+
+    if (transition != null) {
+      compose.setContent {
+        Backstack(
+          backstack.keys.toList(),
+          frameController = rememberTransitionController(
+            animationSpec = animation,
+            transition = transition
+          )
+        ) { BasicText(backstack.getValue(it)) }
+      }
+    } else {
+      compose.setContent {
+        Backstack(
+          backstack.keys.toList(),
+          frameController = NoopFrameController()
+        ) { BasicText(backstack.getValue(it)) }
+      }
     }
     val initialText = if (forward) "one" else "two"
     val newText = if (forward) "two" else "one"