fix(test-runner): update mjs entrypoint with formatError

Author: bashmish

.changeset/fresh-boxes-yawn.md

@@ -0,0 +1,5 @@
+---
+'@web/test-runner': patch
+---
+
+update mjs entrypoint with formatError

packages/test-runner/index.mjs

@@ -7,6 +7,7 @@ const {
   defaultReporter,
   summaryReporter,
   dotReporter,
+  formatError,
   constants,
   TestRunner,
   TestRunnerCli,
@@ -24,6 +25,7 @@ export {
   defaultReporter,
   summaryReporter,
   dotReporter,
+  formatError,
   constants,
   TestRunner,
   TestRunnerCli,