As identified in #2179081: [meta] Inventory search.module tests. File issues to address test holes., there is not really very good testing of the User search functionality provided by the Core Search/User modules.
We need to add tests for the following:
a) Permissions: Verify that users with/without search content/access user profiles permissions can/cannot search users via substrings on user account names, via any configured UserSearch pages via the UI. Verify that users with/without administer users permissions can/cannot search users via email address substrings.
b) Basic functionality: Verify that keyword searching for user account names works via the UI, and that a non-matching query gives an appropriate message.
c) Admin functionality: Verify that keyword searching for user account email addresses works via the UI, and that a non-matching query gives an appropriate message.
Some, but not all, of this is tested in the existing UserSearchTest class under the User module. It just needs to be expanded to make sure all the items are covered.