In v17.0, the test TestUiPublisher.test_course_publisher_elearning_manager of website_slides fails randomly at the step: "eLearning: check uploaded pdf presence and perform comparison" with the error: "Cannot read properties of null (reading 'getAttribute')"

How to reproduce it ?
Locally, TestUiPublisher.test_course_publisher_elearning_manager of website_slides runs without errors but when executing it in the browser we get the same error as the runbot systematically:
–	Install website_slides
–	Go to shell mode and execute the setup of the test –	(Backup DB if you want to run the test multiple time) –	Launch in normal mode and connect with demo user –	Go to URL: /web?debug=assets%2Ctests
–	Type “odoo.startTour('course_publisher_standard');” in the console –	An error happens at step “eLearning: check uploaded pdf presence and perform comparison”: Cannot read properties of null (reading 'getAttribute') (to avoid to wait too long, you can shorten the test by keeping mostly “addPdfToSection”)

The problem occurs because the rendering of the iframe o_wslides_iframe_viewer is not always completed when the check is triggered. We solve this problem by waiting for the rendering to finish before performing the test (using "await contains"). For safety, we do the same for another check as this might cause a similar problem in the future.

Although the test has been introduced in 16.2 (see odoo/odoo#117293) but the error happens only in 17.0 (tested only in 16.4 the way described above), we publish only the correction in 17.0 and further to avoid different correction in various version.

Task-3623736

**Before this commit:**
If you select more than one trigger field on base.automation:
```

File "/home/odoo/src/odoo/17.0/addons/base_automation/models/base_automation.py", line 302, in _compute_trg_field_ref__model_and_display_names relation = automation.trigger_field_ids.relation
File "/home/odoo/src/odoo/17.0/odoo/fields.py", line 1146, in __get__ record.ensure_one()
File "/home/odoo/src/odoo/17.0/odoo/models.py", line 5819, in ensure_one raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: ir.model.fields(<NewId origin=3241>, <NewId origin=3238>)
```

**After this commit:**
`trg_field_ref_model_name` and `trg_field_ref_display_name` are only used for the `base_automation_trigger_reference` widget, which is only used when `trigger in ['on_stage_set', 'on_tag_set']`. We only need to compute them in this case, which will be OK since they use only one trigger field.

opw-3607314

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