Closes mozilla-mobile#4828 Initial tests for FragmentPreDrawMan… (#6134)

* Closes mozilla-mobile#4828 Initial tests for FragmentPreDrawManager

* Closes mozilla-mobile#4828 verify if addOnPreDrawListener was called

app/src/test/java/org/mozilla/fenix/utils/FragmentPreDrawManagerTest.kt

@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.utils
+
+import androidx.core.view.OneShotPreDrawListener
+import androidx.fragment.app.Fragment
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import io.mockk.mockk
+import io.mockk.verify
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.ObsoleteCoroutinesApi
+import kotlinx.coroutines.test.runBlockingTest
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mozilla.fenix.TestApplication
+import org.robolectric.annotation.Config
+
+@ObsoleteCoroutinesApi
+@ExperimentalCoroutinesApi
+@RunWith(AndroidJUnit4::class)
+@Config(application = TestApplication::class)
+class FragmentPreDrawManagerTest {
+    private fun doNothing() { /*noop*/ }
+
+    @Test
+    fun `execute doOnPreDraw`() {
+        runBlockingTest {
+            val fragmentMock = mockk<Fragment>(relaxed = true)
+            val fragmentPreDrawManager = FragmentPreDrawManager(fragmentMock)
+            val listener = OneShotPreDrawListener.add(fragmentMock.view!!) { mockk() }
+
+            verify { fragmentMock.postponeEnterTransition() }
+            fragmentPreDrawManager.execute { doNothing() }
+            verify { fragmentMock.view?.viewTreeObserver?.addOnPreDrawListener(listener) }
+        }
+    }
+}