perf: only inject DOM Testing Library if necessary

If window.TestingLibraryDom exists there is no need to set it up again.
If it does exist it should still be configured to make sure it is using
the correct config.

Author: olivierwilkinson

src/index.ts

@@ -27,7 +27,13 @@ const DOM_TESTING_LIBRARY_UMD = fs
 let _config: Partial<Config>
 
 async function injectDOMTestingLibrary(container: Element) {
-  await container.execute(DOM_TESTING_LIBRARY_UMD)
+  const shouldInject = await container.execute(function () {
+    return !window.TestingLibraryDom
+  })
+
+  if (shouldInject) {
+    await container.execute(DOM_TESTING_LIBRARY_UMD)
+  }
 
   if (_config) {
     await container.execute(function (config: Config) {