Avoid creating too many unnecessary build directories for Fantom bundles (facebook#52788)

Summary:
Pull Request resolved: facebook#52788

Changelog: [internal]

Minor change to just create a single output directory for generated JS code in Fantom per Jest run.

Reviewed By: lenaic

Differential Revision: D78808564

fbshipit-source-id: 70e1a60dfcdcc3fc6ee5f08ced1e9c8f8cab2782

Author: rubennorte

private/react-native-fantom/runner/global-setup/globalSetup.js

@@ -15,6 +15,8 @@ export default async function globalSetup(
   globalConfig: {...},
   projectConfig: {...},
 ): Promise<void> {
+  process.env.__FANTOM_RUN_ID__ ??= `run-${Date.now()}`;
+
   validateEnvironmentVariables();
 
   if (!isOSS) {

private/react-native-fantom/runner/runner.js

@@ -51,11 +51,17 @@ import nullthrows from 'nullthrows';
 import path from 'path';
 import readline from 'readline';
 
+const fantomRunID = process.env.__FANTOM_RUN_ID__;
+if (fantomRunID == null) {
+  throw new Error(
+    'Expected Fantom run ID to be set by global setup, but it was not (process.env.__FANTOM_RUN_ID__ is null)',
+  );
+}
+
 const BUILD_OUTPUT_ROOT = path.resolve(__dirname, '..', 'build', 'js');
-fs.mkdirSync(BUILD_OUTPUT_ROOT, {recursive: true});
-const BUILD_OUTPUT_PATH = fs.mkdtempSync(
-  path.join(BUILD_OUTPUT_ROOT, `run-${Date.now()}-`),
-);
+const BUILD_OUTPUT_PATH = path.join(BUILD_OUTPUT_ROOT, fantomRunID);
+
+fs.mkdirSync(BUILD_OUTPUT_PATH, {recursive: true});
 
 function buildError(
   failureDetail: FailureDetail,