use unstable prefix for wk m.tile_server in tests

[kerryarchibald]

---

This change is marked as an internal change (Task), so will not be included in the changelog.

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[codecov]

Codecov Report

Merging #8316 (d73f510) into develop (27118a9) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

❗ Current head d73f510 differs from pull request most recent head 0e8a6ed. Consider uploading reports for the commit 0e8a6ed to get more accurate results

@@             Coverage Diff             @@
##           develop    #8316      +/-   ##
===========================================
- Coverage    30.01%   29.83%   -0.18%     
===========================================
  Files          879      874       -5     
  Lines        50193    50025     -168     
  Branches     12783    12729      -54     
===========================================
- Hits         15064    14924     -140     
+ Misses       35129    35101      -28     

Impacted Files	Coverage Δ
src/utils/WellKnownUtils.ts	61.76% <100.00%> (ø)
src/autocomplete/EmojiProvider.tsx	16.27% <0.00%> (-72.86%)	⬇️
src/autocomplete/AutocompleteProvider.tsx	0.00% <0.00%> (-60.72%)	⬇️
src/utils/EventUtils.ts	38.20% <0.00%> (-4.51%)	⬇️
src/components/structures/ScrollPanel.tsx	34.47% <0.00%> (-3.71%)	⬇️
src/components/views/elements/AccessibleButton.tsx	34.21% <0.00%> (-2.38%)	⬇️
...ews/settings/tabs/room/SecurityRoomSettingsTab.tsx	0.00% <0.00%> (-0.76%)	⬇️
src/components/structures/MessagePanel.tsx	64.40% <0.00%> (-0.43%)	⬇️
src/utils/permalinks/Permalinks.ts	74.31% <0.00%> (-0.12%)	⬇️
src/components/views/dialogs/InviteDialog.tsx	0.51% <0.00%> (-0.05%)	⬇️
... and 27 more

[turt2live]

thanks - my panic was that we didn't even have an UnstableValue for it, so good to see we have that :D