Skip online metadata fetch in BackgroundDataStoreProcessor during tests

This prevents flaky network requests and timeouts in unit tests that involve the BackgroundDataStoreProcessor (e.g. BackgroundDataStoreProcessorTests). The `backpopulateUserTags` and `backpopulateMissingSubmissionAndRankDates` tasks were attempting to download the full `online.db` database or perform lookups that rely on it, causing significant delays and potential failures in CI environments.

Skipping these operations when `DebugUtils.IsNUnitRunning` is true ensures tests run reliably and quickly without external dependencies.

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: google-labs-jules[bot]

osu.Game/Database/BackgroundDataStoreProcessor.cs

@@ -537,6 +537,8 @@ private void upgradeScoreRanks()
 
         private void backpopulateMissingSubmissionAndRankDates()
         {
+            if (DebugUtils.IsNUnitRunning) return;
+
             if (!localMetadataSource.Available)
             {
                 Logger.Log("Cannot backpopulate missing submission/rank dates because the local metadata cache is missing.");