Add a fail string for app pair menu tests

This patch adds a clarifying message for testSaveAppPairMenuItemExistsOnSplitPair() and testSaveAppPairMenuItemDoesNotExistOnSingleTask(). The test should not run when the feature is not yet enabled.

Fixes: 297083938
Test: N/A
Change-Id: Idf4c99ed393e2d6e2c2b7b77dc6083a27c4f90d1