fix: flaky tests in PreviewerViewModelTest

Fixes BackendCollectionAlreadyOpenException

init { } ran code, but the coroutines were not
 cancelled

Fixes 20704

Author: david-allison

AnkiDroid/src/test/java/com/ichi2/anki/previewer/PreviewerViewModelTest.kt

@@ -16,6 +16,7 @@
 package com.ichi2.anki.previewer
 
 import androidx.lifecycle.SavedStateHandle
+import androidx.lifecycle.viewModelScope
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.ichi2.anki.Flag
 import com.ichi2.anki.browser.IdsFile
@@ -26,9 +27,11 @@ import io.mockk.coEvery
 import io.mockk.every
 import io.mockk.mockk
 import io.mockk.spyk
+import kotlinx.coroutines.cancel
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.test.TestScope
 import kotlinx.coroutines.test.advanceUntilIdle
+import org.junit.After
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
 import org.junit.Assert.assertTrue
@@ -84,6 +87,12 @@ class PreviewerViewModelTest : JvmTest() {
         coEvery { viewModel.prepareCardTextForDisplay(any()) } answers { firstArg() }
     }
 
+    @After
+    override fun tearDown() {
+        viewModel.viewModelScope.cancel()
+        super.tearDown()
+    }
+
     @Test
     fun `next button`() =
         runTest {