Add unit and integration tests for 7 remaining test issues

[cicalese]

Summary

Unit tests (27 new test cases)

• extractSecretKey (6 tests): already in .env, from LocalSettings, from CommonSettings, generate new, dry run (Test: add unit tests for extractSecretKey migration #611)
• fixVectorDefaultSkin (4 tests): buggy content fixed, correct content unchanged, missing file, dry run (Test: add unit tests for fixVectorDefaultSkin migration #613)
• getWikiIDsForRestore (4 tests): multiple wikis, single wiki, empty, unknown wiki (Test: add unit tests for getWikiIDsForRestore #615)
• restoreUserDir (4 tests): symlink replaced, regular dir unchanged, missing dir created, symlink without source (Test: add unit tests for restoreUserDir #617)
• WriteIDEConfigs (4 tests + subtests): VSCode + PhpStorm configs created, overwrite, defaults, xdebug port (Test: add unit tests for WriteIDEConfigs #618)

Integration tests (2 new tests)

• TestFarm_AddAndRemoveWiki: create instance, add wiki, verify both respond, remove wiki, verify cleanup (Integration test: wiki farm lifecycle (add and remove) #620)
• TestExtensionSkin_EnableDisable: enable/disable Cite extension and Timeless skin, verify via siteinfo API (Integration test: extension and skin enable/disable #619)

Fixes #611, #613, #615, #617, #618, #619, #620

Test plan

• go test ./... passes (unit tests)
• go vet -tags integration ./tests/integration/ passes
• gofmt clean
• Integration tests pass in CI

[yaronkoren]

Great!