Fix overlay's misalignment that contain merge cells

[budnix]

Context

The PR fixes the overlay's misalignment for overlays that contain merge cells. The reason for the misalignment was the incorrect height of the first row, which is the only one that receives the border-width attribute of 1px for the top, which causes the content in the cells to be pushed by 1px (as the box-sizing is set as border-box). The fix compensates for that 1px in CSS.

How has this been tested?

I tested the changes locally, and I updated the tests according to the changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Related issue(s):

1. fixes https://github.com/handsontable/dev-handsontable/issues/2252

Affected project(s):

• handsontable

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement

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