Fixes #39482 - Replace enzyme test in UserDropdown with RTL

[kfamilonidis]

Fixes #39482 - Replace enzyme test in HeaderToolbar, UserDropdown with…

[Lukshio]

Summary

Reasonable migration; the new "hides impersonate icon when not impersonated" test in HeaderToolbar.test.js is a good addition, and UserDropdowns.test.js is a clean, focused migration. However, mocking out nearly every child of HeaderToolbar drops some prop-wiring coverage the old snapshot had.

Critical (must fix before merge)

• None

Suggestions (should fix)

• The removed snapshot verified that TaxonomySwitcher received the correct locations/organizations arrays. The new mock (() => <div data-testid="taxonomy-switcher" />) ignores props entirely, so that data-flow coverage is lost. Consider surfacing the received props in the mock (similar to how the UserDropdowns/ImpersonateIcon mocks already render user.current_user.name / stopImpersonationUrl) so this regression class is still caught.
• Commit message uses Fixes #39482: ... - the documented convention (see develop history, e.g. Fixes #38653 - Fix settings show endpoint) is Fixes #XXXXX - description with a hyphen, not a colon.

Nice to have (optional)

• None

Checklist gaps

• TaxonomySwitcher prop-wiring coverage lost in the migration (see above).
• Commit message hyphen convention.

[kfamilonidis]

@Lukshio The TaxonomySwitcher mock now surfaces its received props.

[andreilakatos]

@Lukshio @kfamilonidis
Should be rebased, I created this PR for HeaderToolbar that was merged
#11198

Will link this PR to the UserDropdown ticket

[Lukshio]

ack, passes locally, Thanks @kfamilonidis @andreilakatos