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

@@ -58,8 +58,18 @@ export default class LightRunner {
             { transferList: [mc.port1] }
           )
           .then(
-            result => void onResult(test, result),
-            error => void onFailure(test, error)
+            result => {
+              // When `--runInBand` is enabled, `InBandTinypool` executes tasks
+              // in a microtask which may complete prior to having received the
+              // start message on the `MessageChannel`. As such, disconnect the
+              // start callback to avoid late invocations.
+              mc.port2.onmessage = null;
+              onResult(test, result);
+            },
+            error => {
+              mc.port2.onmessage = null;
+              onFailure(test, error);
+            }
           );
       })
     );