When serializing to OLX, the Learning Core runtime wraps HTML content in
CDATA to avoid having to escape every individual `<`, `>`, and `&`. The
runtime also puts newlines around the content within the CDATA,
So, given HTML content `...`, we get `<![CDATA[\n...\n]]>`.
The problem is that every time you serialize an HTML block to OLX, it
adds another pair of newlines. These newlines aren't visible to the end
users, but they do make it so that importing and exporting content never
reached a stable, aka "canonical" form. It also makes unit testing
difficult, because the value of `html_block.data` becomes a moving
target.
We do not believe these newlines are necessary, so we have removed them
from the `CDATA` block, and added a unit test to ensure that HTML blocks
having a canonical serialization.
Closes: https://github.com/openedx/edx-platform/issues/35525
f65403975a