Fix coverage plugin not writing output - use recorder for async generation

- Coverage generation was async but not awaited by event.all.after
- Process could exit before coverage file was written
- Now uses recorder.add() to ensure coverage generation completes
- Fixes 'writing output/coverage' message not appearing in tests

Author: kobenguyent

lib/plugin/coverage.js

@@ -149,9 +149,17 @@ export default function (config) {
   const coverageReport = new CoverageReport(coverageOptions)
   coverageReport.cleanCache()
 
-  event.dispatcher.on(event.all.after, async () => {
-    output.print(`writing ${coverageOptions.outputDir}`)
-    await coverageReport.generate()
+  event.dispatcher.on(event.all.after, () => {
+    // Add coverage generation to recorder to ensure it completes before process exit
+    recorder.add(
+      'generate coverage report',
+      async () => {
+        output.print(`writing ${coverageOptions.outputDir}`)
+        await coverageReport.generate()
+      },
+      true,
+      false,
+    )
   })
 
   //  we're going to try to "start" coverage before each step because this is