Fix non-deterministic test failure in BackgroundIndexingTests.testPrepareTarget

Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

@@ -554,27 +554,30 @@ final class BackgroundIndexingTests: XCTestCase {
       DidChangeWatchedFilesNotification(changes: [FileEvent(uri: try project.uri(for: "MyFile.swift"), type: .changed)])
     )
 
-    let diagnosticRefreshReceived = self.expectation(description: "Received diagnostic refresh request")
-    project.testClient.handleNextRequest { (_: DiagnosticsRefreshRequest) in
-      diagnosticRefreshReceived.fulfill()
-      return VoidResponse()
-    }
-
     // Send a document request for `uri` to trigger re-preparation of its target. We don't actually care about the
     // response for this request. Instead, we wait until SourceKit-LSP sends us a `DiagnosticsRefreshRequest`,
     // indicating that the target of `uri` has been prepared.
     _ = try await project.testClient.send(
       DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
     )
-    try await fulfillmentOfOrThrow([diagnosticRefreshReceived])
 
-    let updatedDiagnostics = try await project.testClient.send(
-      DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
-    )
-    guard case .full(let updatedDiagnostics) = updatedDiagnostics else {
-      XCTFail("Expected full diagnostics")
-      return
+    let receivedEmptyDiagnostics = self.expectation(description: "Received diagnostic refresh request")
+    project.testClient.handleNextRequest { (_: DiagnosticsRefreshRequest) in
+      Task {
+        let updatedDiagnostics = try await project.testClient.send(
+          DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
+        )
+        guard case .full(let updatedDiagnostics) = updatedDiagnostics else {
+          XCTFail("Expected full diagnostics")
+          return
+        }
+        if updatedDiagnostics.items.isEmpty {
+          receivedEmptyDiagnostics.fulfill()
+        }
+      }
+      return VoidResponse()
     }
-    XCTAssertEqual(updatedDiagnostics.items, [])
+
+    try await fulfillmentOfOrThrow([receivedEmptyDiagnostics])
   }
 }