Add assertions to PiP related test cases in DesktopTasksControllerTest

Transaction is not examined actually before this CL.
This CL adds assertions and also updates the test name according to the
current behavior.

Bug: 396215361
Flag: EXEMPT test only change
Test: WMShellUnitTests
Change-Id: Ifa2cdb6d2146ba3ed7ec9c3e3833094fb5ae7bf1