Introduce simple SuggestionsPopupWindowTest.

This CL introduces very simple test case for SuggestionPopupWindow.
I'm going to introduce more test cases for complex scenario once this
approach is accepted, e.g. multiple suggestion span and misspelled flag.

This CL is a part of groundwork for Bug 15347319 and no user visible
change is intended with this CL.

Bug: 15347319
Change-Id: Idf166cbecb3e33be213e4104cf1afd827906f2ad