[gap-decoration] Fix 2 flex tests that were not testing as intended

This CL fixes two flex tests that were meant to test that we paint
gaps confirming to the collapsing border model (more info on the
original CL:6442638). It seems that these tests were wrong, they were
set up incorrectly and didnt even have a row/column gap.

This CL fixes them so they are actually testing what we want them
to test.

Bug: 357648037
Change-Id: I2f13a07b9b63299b3f3c589d32cd48e7c103133c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6995363
Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1522567}