[fenix] Closes https://github.com/mozilla-mobile/fenix/issues/21335: return self instead of mock in FenixSnackbarKtTest.

This is a speculative fix for the intermittent issue. Typically, these
intermittents are caused by mocked lambdas but there is no mocked lambda
here. If this doesn't work, one `any()` argument fills in for a lambda:
it's possible that's causing the failure.

Unfortunately, I can't verify this fix easily because the "run test until
failure" option was removed from Android Studio.

See d396c9eb41298cc07fbf136f1de971010bc45d97 for a prior attempt to
address the intermittents in this class.

app/src/test/java/org/mozilla/fenix/tabstray/ext/FenixSnackbarKtTest.kt

@@ -1,3 +1,7 @@
+/* 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.tabstray.ext
 
 import android.content.Context
@@ -74,7 +78,7 @@ class FenixSnackbarKtTest {
 
         every { snackbar.setAnchorView(anchor) }.answers { snackbar }
         every { snackbar.view }.answers { view }
-        every { snackbar.setAction(any(), any()) }.answers { mockk(relaxed = true) }
+        every { snackbar.setAction(any(), any()) }.answers { snackbar }
         every { snackbar.anchorView }.answers { anchor }
 
         snackbar.anchorWithAction(anchor, {})