test: add acceptance tests for the update of an endpoint

[lriberon]

Description

The modification of an existing endpoint resource has been tested manually and an Acceptance test has been created for it.

How to test?

Run Acceptance tests

Run the make testacc command to run all the Acceptance Tests.

Manually

1. Create a new endpoint resource (you can follow the steps provided in feat: add validations func & create acceptance tests #32 to create a new endpoint resource)
2. Update the fields you want on this resource
3. Check the information to be updated: terraform plan
4. An error should be raised if there is a wrong input (e.g. wrong source value)
5. If no error has been raised, you can apply your changes: terraform apply. You should see the output: `Apply complete! Resources: 0 added, 1 changed, 0 destroyed.
6. Check on the dashboard that the information are correctly updated