Download langtags with etag

[imnasnainaec]

Fixes #987 following #987 (comment)
Continues #1232 where @elisunger left off

---

This change is 

[github-actions]

Palaso Tests

     4 files  ± 0       4 suites  ±0   13m 53s ⏱️ -36s
 4 884 tests + 4   4 650 ✅  - 2  233 💤 + 5  1 ❌ +1 
15 906 runs  +16  15 198 ✅  - 8  704 💤 +20  4 ❌ +4 

For more details on these failures, see this check.

Results for commit e631bae. ± Comparison against base commit eec9b0b.

This pull request removes 2 and adds 6 tests. Note that renamed tests count towards both.

SIL.WritingSystems.Tests.SldrTests ‑ LanguageTags_NewerCachedLangTagsSldrOffline_UseCachedLangTags
SIL.WritingSystems.Tests.SldrTests ‑ LanguageTags_OlderCachedLangTagsSldrOffline_UseEmbeddedLangTags

SIL.WritingSystems.Tests.SldrTests ‑ DownloadLangTags_ExistingETagNoFile
SIL.WritingSystems.Tests.SldrTests ‑ DownloadLangTags_NoPreviousETagExistingFile
SIL.WritingSystems.Tests.SldrTests ‑ DownloadLangTags_NoPreviousETagNoFile
SIL.WritingSystems.Tests.SldrTests ‑ DownloadLangTags_NotModified
SIL.WritingSystems.Tests.SldrTests ‑ DownloadLangTags_Outdated
SIL.WritingSystems.Tests.SldrTests ‑ LanguageTags_SldrOffline_UseCachedLangTags

♻️ This comment has been updated with latest results.