Wait for indexstore-db to update after building a SwiftPM project in tests

Otherwise we can get into race conditions where we run a request before the index has actually been updated.

While at it, also add logging when IndexStoreDB has been updated.

rdar://126635046

Sources/SourceKitLSP/SourceKitIndexDelegate.swift

@@ -12,6 +12,7 @@
 
 import Dispatch
 import IndexStoreDB
+import LSPLogging
 import SKCore
 import SKSupport
 
@@ -62,6 +63,7 @@ public actor SourceKitIndexDelegate: IndexDelegate {
   }
 
   private func indexChanged() {
+    logger.debug("IndexStoreDB changed")
     for callback in mainFilesChangedCallbacks {
       queue.async {
         await callback()

Tests/SourceKitLSPTests/RenameTests.swift

@@ -584,7 +584,7 @@ final class RenameTests: XCTestCase {
           name: "MyLibrary",
           targets: [
             .target(
-              name: "MyLibrary", 
+              name: "MyLibrary",
               swiftSettings: [.unsafeFlags(["-Xfrontend", "-disable-objc-attr-requires-foundation-module"])]
             )
           ]
@@ -635,7 +635,7 @@ final class RenameTests: XCTestCase {
           name: "MyLibrary",
           targets: [
             .target(
-              name: "MyLibrary", 
+              name: "MyLibrary",
               swiftSettings: [.unsafeFlags(["-Xfrontend", "-disable-objc-attr-requires-foundation-module"])]
             )
           ]
@@ -1158,6 +1158,7 @@ final class RenameTests: XCTestCase {
     )
 
     try await SwiftPMTestProject.build(at: project.scratchDirectory)
+    _ = try await project.testClient.send(PollIndexRequest())
 
     let resultAfterFileMove = try await project.testClient.send(
       RenameRequest(textDocument: TextDocumentIdentifier(callerUri), position: callerPositions["3️⃣"], newName: "bar")

Tests/SourceKitLSPTests/TestDiscoveryTests.swift

@@ -247,6 +247,7 @@ final class TestDiscoveryTests: XCTestCase {
     )
 
     try await SwiftPMTestProject.build(at: project.scratchDirectory)
+    _ = try await project.testClient.send(PollIndexRequest())
 
     // After indexing, we know that `LooksLikeTestCaseButIsNot` does not inherit from `XCTestCase` and we don't report any tests.
     let indexBasedTests = try await project.testClient.send(