Remove PySide6-related test skips

[mdickinson]

We're currently skipping some tests in our Ubuntu GitHub Actions runs because of a PySide6 bug that triggers an end-of-process segfault. Those test skips should be removed once they're no longer needed. This requires:

• A PySide 6.4.3 release (already happened)
• EDM to have PySide 6.4.3 or later
• Pyface fixes for compatibility with PySide6 6.4.x. (Fixes already made; now waiting on a Pyface release.)

xref: #476
xref: https://bugreports.qt.io/browse/PYSIDE-2254

[mdickinson]

#528 is related

[mdickinson]

"EDM to have PySide 6.4.3 or later" <- This condition isn't actually strictly required, since the next release of Pyface contains a workaround for the segfaults that should be good enough for us to be able to remove the skips.

So #528 and this issue can likely be solved together as soon as the next Pyface release is made.