The tests rely on scatter mocks `scatter_fill_self_next`
and `scatter_fill_corner_cases` and test for a number of points
that are currently not correctly detected when hovered over
for tonexty and tozeroy fills.

Previous code would not correctly construct the polygons
used for detection whether the cursor is within the fill
region of a trace.

Some of the previously created tests fail with this as the
hover points in question cannot currently be correctly
detected using the detection polygons.

…ests in scatter plots.

However, SVGElement does not allow for an easy way to determine the positioning of
the hover label, so the polygons are still in use for that.

For curved edges of fills there is a chance that the hover detection polygons miss a point and the correct hover label position cannot
be determined. Previous code fell back to positioning the label
at the largest edge of any polygon of the trace of their combined
vertical midpoint, with undetermined behaviour if no polygon
overlapped the midpoint.
This change instead falls back to simply displaying the label at the
current cursor position, which simplifies code and results in less
confusing label placement.

…catter plot fill tonext* plotly#6865"

Co-authored-by: Mojtaba Samimi <[email protected]>