IndexEventHandlerTest: Fix 'Unlikely argument type' Eclipse warnings

Eclipse 4.7.0 (Oxygen) introduced a new compiler warning:

  Unlikely argument type

In this case the warning is being emitted because a String is being
passed in place of Index{Account|Change|Group}Task instance.

Replace these with dummy instances of the expected class.

Change-Id: I2c44501c4e0da63f88ad22d1a4a91ff55bed025c