MainThreadMonitor: don't crash if there is no test in progress

Tests/BackendIntegrationTests/MainThreadMonitor.swift

@@ -44,9 +44,13 @@ final class MainThreadMonitor {
                 let deadline = DispatchTime.now() + Self.threshold + Self.checkInterval
                 let result = semaphore.wait(timeout: deadline)
 
+                // `XCTest` sometimes blocks the main thread at the end of a test.
+                // We don't want to detect that as a deadlock.
+                let timedOutDuringTest = CurrentTestCaseTracker.shared.testInProgress && result == .timedOut
+
                 precondition(
-                    result != .timedOut,
-                    "Main thread was blocked for more than \(Self.threshold.seconds) seconds"
+                    !timedOutDuringTest,
+                    "Main thread was blocked for more than \(Self.threshold.seconds) seconds during a test"
                 )
             }
         }

Tests/UnitTests/TestHelpers/CurrentTestCaseTracker.swift

@@ -28,6 +28,10 @@ final class CurrentTestCaseTracker: NSObject, XCTestObservation {
         currentTestCase = nil
     }
 
+    var testInProgress: Bool {
+        return self.currentTestCase != nil
+    }
+
     /// Extracts the name of the current running test.
     ///
     /// Example: extracts `testLoginCachesForSameUserIDs`