Avoid late start message after test has finished in --runInBand mode

The start message that is sent over a test's MessageChannel may arrive after
the tests' Promise has already resolved, i.e. the finished callback has
already been executed. This results in excessive status message logging,
as the late start event does activate the test as "running" even though
it has already finished, rendering it in the "running" state forever (and
consequently generating excessive status messages as more and more tests become
considered "running").

Fixes #76

src/index.js

@@ -48,8 +48,16 @@ export default class LightRunner {
 
     return Promise.all(
       tests.map(test => {
+        // When `--runInBand` is enabled, `InBandPiscina` executes tasks in a microtask which may complete prior to
+        // having received the start message on the `MessageChannel`. As such, the start callback should not be invoked
+        // after the completion callbacks have already been invoked.
+        let finished = false;
         const mc = new MessageChannel();
-        mc.port2.onmessage = () => onStart(test);
+        mc.port2.onmessage = () => {
+          if (!finished) {
+            onStart(test);
+          }
+        };
         mc.port2.unref();
 
         return this._piscina
@@ -58,8 +66,14 @@ export default class LightRunner {
             { transferList: [mc.port1] }
           )
           .then(
-            result => void onResult(test, result),
-            error => void onFailure(test, error)
+            result => {
+              finished = true;
+              onResult(test, result);
+            },
+            error => {
+              finished = true;
+              onFailure(test, error);
+            }
           );
       })
     );