Improve the testcases of filePicker

[Santosh]

UbuntuWebView filepicker testing is quite basic, it just test launch/close of filepicker with some property matching. Whole file selection success/failure is not tested since there is not clear method available to do that( simulating the content-hub file selection behavior)

There has been grey information that content-hub is planning to add support for that in future, and
may be querying with dbus might be helpful.

I plan to look into this.