Tests: Fix draggable_options expected test count to be dynamic

The test count was previously incremented to 8 when normally 6 tests are run.  After figuring out the dynamic test code, I just made the expect depend on the same array length

Also re-enabled the test in the Gruntfile since it now passes.

(fwiw, I see why this might have been done, positions had 'static' added, but the loop is over scrollPositions ; @mikesherov might know if there was expected to do more tests.. )