[test] Fix notification handler test when run in serial

The test was passing with --parallel since each test was being run in
its own process, but not when run in serial since other documents could
trigger unexpected notifications. Also make the uniqueness of the path
reliable using a mutable workspace.

Author: benlangmuir

Package.swift

@@ -126,6 +126,7 @@ let package = Package(
         dependencies: [
           "SourceKitD",
           "SKCore",
+          "SKTestSupport",
         ]
       ),
 

Tests/SourceKitDTests/SourceKitDTests.swift

@@ -14,6 +14,8 @@ import SourceKitD
 import SKCore
 import TSCBasic
 import TSCUtility
+import ISDBTibs
+import ISDBTestSupport
 import XCTest
 
 final class SourceKitDTests: XCTestCase {
@@ -31,26 +33,38 @@ final class SourceKitDTests: XCTestCase {
   }
 
   func testMultipleNotificationHandlers() {
+    let ws = try! mutableTibsTestWorkspace(name: "proj1")!
     let sourcekitd = try! SourceKitDImpl.getOrCreate(dylibPath: SourceKitDTests.sourcekitdPath)
     let keys = sourcekitd.keys
+    let path: String = ws.testLoc("c").url.path
+
+    let isExpectedNotification = { (response: SKDResponse) -> Bool in
+      if let notification: sourcekitd_uid_t = response.value?[keys.notification],
+         let name: String = response.value?[keys.name]
+      {
+        return name == path && notification == sourcekitd.values.notification_documentupdate
+      }
+      return false
+    }
 
     let expectation1 = expectation(description: "handler 1")
     let handler1 = ClosureNotificationHandler { response in
-      XCTAssertEqual(response.value?[keys.notification], sourcekitd.values.notification_documentupdate)
-      expectation1.fulfill()
+      if isExpectedNotification(response) {
+        expectation1.fulfill()
+      }
     }
     sourcekitd.addNotificationHandler(handler1)
 
     let expectation2 = expectation(description: "handler 2")
     let handler2 = ClosureNotificationHandler { response in
-      XCTAssertEqual(response.value?[keys.notification], sourcekitd.values.notification_documentupdate)
-      expectation2.fulfill()
+      if isExpectedNotification(response) {
+        expectation2.fulfill()
+      }
     }
     sourcekitd.addNotificationHandler(handler2)
 
     let req = SKDRequestDictionary(sourcekitd: sourcekitd)
     req[keys.request] = sourcekitd.requests.editor_open
-    let path: String = #file
     req[keys.name] = path
     req[keys.sourcetext] = """
       func foo() {}