test(language-service): Make project service a singleton (#39308)

Constructing a project service is expensive. Making it a singleton could
speed up tests considerably.

On my MacBook Pro, test execution went from 24.4s to 14.5s (~40% improvement).

PR Close #39308

packages/language-service/ivy/test/mock_host.ts

@@ -69,6 +69,12 @@ export const host: ts.server.ServerHost = {
   },
 };
 
+/**
+ * Constructing a project service is expensive (~2.5s on MacBook Pro), so it
+ * should be a singleton service shared throughout all tests.
+ */
+let projectService: ts.server.ProjectService;
+
 /**
  * Create a ConfiguredProject and an actual program for the test project located
  * in packages/language-service/test/project. Project creation exercises the
@@ -76,17 +82,19 @@ export const host: ts.server.ServerHost = {
  * and modify test files.
  */
 export function setup() {
-  const projectService = new ts.server.ProjectService({
-    host,
-    logger,
-    cancellationToken: ts.server.nullCancellationToken,
-    useSingleInferredProject: true,
-    useInferredProjectPerProjectRoot: true,
-    typingsInstaller: ts.server.nullTypingsInstaller,
-  });
-  // Opening APP_COMPONENT forces a new ConfiguredProject to be created based
-  // on the tsconfig.json in the test project.
-  projectService.openClientFile(APP_COMPONENT);
+  if (!projectService) {
+    projectService = new ts.server.ProjectService({
+      host,
+      logger,
+      cancellationToken: ts.server.nullCancellationToken,
+      useSingleInferredProject: true,
+      useInferredProjectPerProjectRoot: true,
+      typingsInstaller: ts.server.nullTypingsInstaller,
+    });
+    // Opening APP_COMPONENT forces a new ConfiguredProject to be created based
+    // on the tsconfig.json in the test project.
+    projectService.openClientFile(APP_COMPONENT);
+  }
   const project = projectService.findProject(TSCONFIG);
   if (!project) {
     throw new Error(`Failed to create project for ${TSCONFIG}`);