[FIX] web_editor: fix html table paste tests

Those tests happened to fail nondeterministically for no apparent
reason and with multiple different wrong tests results.

Some of those wrong results were particularly striking like having
completely empty contents, while others looked more like some kind
of race condition.

Aiming to kill two birds with one stone, I moved the html contents
of the tests into their very own test file so that no asynchronous
file loading on the server is done during testing at runtime anymore.

closes odoo/odoo#125020

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>