There was an undocumented change in 5.0 where the shape keys now start with a weight of 1.0 instead of 0.0 when created.
I have added an assert in the shape key test to make sure that the values are set to `0.0`.
This also fixes the assertion in the poll function (asserts should never be in poll functions).
This was caused by a regression caused by 29831d7f09.
The test for importing extra UVs has been updated to check that the data is different between the different UV layers.
