test: fix mocking in RouteUnifyingServiceInitListenerTest

[mcollovati]

Fixes nightly build failures

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.72%. Comparing base (d3e74e9) to head (e84adb3). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2877 +/- ## ======================================= Coverage 92.72% 92.72% ======================================= Files 83 83 Lines 2845 2845 Branches 728 728 ======================================= Hits 2638 2638 Misses 156 156 Partials 51 51 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2877/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/vaadin/hilla/pull/2877/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `92.72% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin#carryforward-flags-in-the-pull-request-comment) to find out more.

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud