Make the Who Is Online label plain text in every language

[avandenberghe]

Closes #199.

Problem
In all 14 language packs, VIEWING_RECENT_TOPICS was 'Viewing <a href="%s">Recent Topics</a>'. But listener::viewonline_overwrite_location() calls lang() without an argument, and core's viewonline_body.html already wraps the location in <a href="{U_FORUM_LOCATION}">. Who Is Online therefore rendered a link nested inside core's link, pointing at a literal %s.

Fix
Drop the markup and placeholder from VIEWING_RECENT_TOPICS in ar, cs, de, de_x_sie, en, es, es_x_tu, fr, nl, pt, ru, sk, sv and uk, keeping each translation's own wording. location_url still supplies the link.

Tests

• New tests/language/viewonline_lang_test.php, documented in tests/tests.md, loads every language pack and checks that the label has no < and no %. It failed for all 14 packs before the fix and passes for all 14 after.
• Unit suite: 69 tests, 0 failures, run locally. The functional tests run in CI.

Checked on the local board (boot script: the listener's real viewonline handler for an app.php/rt session, using phpBB's language service, with the cell built as in prosilver's viewonline_body.html):

before: <td class="info"><a href="…/app.php/rt">Viewing <a href="%s">Recent Topics</a></a></td>
after:  <td class="info"><a href="…/app.php/rt">Viewing Recent Topics</a></td>

No changelog entry or version bump, per this repo's convention.

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