Tests: check existing username case-insensitive

[ptamarit]

Fixes https://github.com/zenodo/ops/issues/326

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Description

Usernames are enforced to be unique in the DB regardless of their case (see related DB column and constraint definition).

This change applies the same validation at the registration of a user to avoid hitting the DB constraint which is then not well handled by Flask (users end up seeing an nginx 502 error).

This is a test-only pull request related to inveniosoftware/invenio-accounts#476.
Here we only improve the test to make sure that invenio-accounts properly finds by usernames case-insensitively.

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[ptamarit]

This is now a test-only pull request (see updated description).
It will turn green as soon as a new version of invenio-accounts containing inveniosoftware/invenio-accounts#476 is released.

• One question: should I bump the minimum version of invenio-accounts from >=2.0.0 to >=5.0.x, since test would fail if using a smaller version?
• And if yes, does it mean that we should also release a version of invenio-userprofiles due to the dependency requirement change?