Give LinkTest and TagTest the same generous HTTP timeout as AbstractBaseTest

[fliebe92]

Summary

• LinkTest and TagTest create and delete real tickets (with articles, links, etc.) for every single test, but their setUpBeforeClass() predates EnvConfigTrait/AbstractBaseTest and never overrides the client's 5 second default HTTP timeout.
• Under load, ticket deletion cascades through enough related records that it can occasionally exceed 5 seconds, causing a ConnectException in tearDown() that gets misattributed to whatever test happened to be running at the time (observed as e.g. LinkTest::testRemoveWithoutTargetTicket failing with a cURL timeout on a DELETE .../tickets/:id call, even though that test's own assertion doesn't touch the network at all).
• AbstractBaseTest already fixed this for the test classes that extend it, by bumping the timeout to 30 seconds (configurable via ZAMMAD_PHP_API_CLIENT_UNIT_TESTS_TIMEOUT). This PR just brings LinkTest and TagTest in line with that existing convention.

Test plan

• CI run of the full test suite should pass as before, just with a more generous timeout available for the create/delete-heavy LinkTest/TagTest fixtures.