Fix StaffType::operator==

For TAB_STAFF_GROUP, the equality operator does not check all the member variables (does not call isSameStructure())

Also, simplified some duplicate tests.
Fix GuitarPro tests (actually revert them to pre-StaffType-refactoring)

Note: since the recent re-factoring of StaffType, all the GuitarPro tests were loading the wrong tablature template (as the == test in Staff::setStaffStyle() was wrong) and have been too quickly updated to accept this.

This reverts the tests to the status before the StaffType re-factoring.
Merge pull request #910 from mgavioli/Fix_StaffType_==_operator

Fix StaffType::operator==