[DO NOT MERGE][6.4.x] Revert 79fc76e8 to see the impact on js tests

[echarles]

DO NOT MERGE

This is a PR to investigate on CI failures on 6.4.x branch as reported on #6319

Temporary and partially revert this commit 79fc76e to see the impact on CI.

Workflow history (https://github.com/jupyter/notebook/actions/workflows/js.yml?page=8 today, link will be soon obsolete) is showing Linux js tests after the fix for GHSA-hwvq-6gjx-j797

[echarles]

This temporary change shows that the casperjs can find the notebook page and that the base tests succeed. As this test suite has not been maintained since 8 months, some tests are failing (eg. expected 3, actual 2)... Those failing tests can be fixed one by one. Before fixing them, we need to find a way to make casperjs operational again with 6.4.x branch with a correct sanitizer that honor GHSA-hwvq-6gjx-j797

cc/ @afshin @blink1073 @Zsailer

Test group: base
Test file: /home/runner/work/notebook/notebook/notebook/tests/base/highlight.js 
PASS highlight: gfm as gfm
PASS highlight: python as python
PASS highlight: ipython as ipython
PASS highlight: ipythongfm as ipythongfm
PASS highlight: .md as text/x-markdown
PASS highlight: text/x-markdown as text/x-markdown
PASS highlight: .py as text/x-python
PASS highlight: Python as text/x-python
PASS highlight: text/x-python as text/x-python
PASS highlight: json as application/json
PASS highlight: JSON as application/json
PASS highlight: application/json as application/json
PASS highlight: json-ld as application/ld+json
PASS highlight: JSON-LD as application/ld+json
PASS highlight: application/ld+json as application/ld+json
PASS highlight: .rb as text/x-ruby
PASS highlight: ruby as text/x-ruby
PASS highlight: Ruby as text/x-ruby
PASS highlight: text/x-ruby as text/x-ruby
PASS !highlight: .pyc errors
PASS !highlight: ../ errors
PASS !highlight: // errors

...
FAIL start_session: 3 events were triggered
#    type: assertEquals
#    file: /home/runner/work/notebook/notebook/notebook/tests/services/session.js
#    subject: 2
#    expected: 3
Running tests with notebook directory '/tmp/tmp7mld0ste'
...

[echarles]

For now 100% of the JS tests are failing on latest 6.4.x branch at this code block (.CodeMirror-code can never be found - With the rever of this PR, it can be found)

notebook/notebook/tests/util.js

Lines 30 to 35 in 3c54a9a

	this.waitForPopup('');
	this.withPopup('', function () {this.waitForSelector('.CodeMirror-code');});
	this.then(function () {
	this.open(this.popups[0].url);
	});

[echarles]

Solution proposed in #6354. Closing.