Fix #130 - Error in test logs

I narrowed down these tests to aborting in a Koa app. This happened
because:

1. Koa listens for the error event on the HTTP stream itself, and
   re-emits it on the app's own event emitter.
2. If there is no listener for an app error, Koa will deviate from
   the node default of crashing, and will instead log the error.
3. This particular error originates from within the HTTP stream,
   causing Koa to log the error message.

I fixed this by actually _expecting_ one error to be emit from the
app, adding an assertion to test this.

src/processRequest.test.mjs

@@ -608,7 +608,7 @@ t.test('Handles unconsumed uploads.', async t => {
   })
 })
 
-t.test('Aborted request.', async t => {
+t.only('Aborted request.', async t => {
   const sendRequest = (port, requestHasBeenReceived) =>
     new Promise((resolve, reject) => {
       const body = new FormData()
@@ -720,7 +720,7 @@ t.test('Aborted request.', async t => {
     }
 
     await t.test('Koa middleware.', async t => {
-      t.plan(5)
+      t.plan(6)
 
       let requestHasBeenReceived
       const requestHasBeenReceivedPromise = new Promise(
@@ -757,6 +757,9 @@ t.test('Aborted request.', async t => {
           finish()
         })
 
+      let appErrors = 0
+      app.on('error', () => appErrors++)
+
       const port = await startServer(t, app)
 
       await sendRequest(port, requestHasBeenReceivedPromise)
@@ -771,6 +774,8 @@ t.test('Aborted request.', async t => {
       if (!fileB.capacitor.closed)
         await new Promise(resolve => fileB.capacitor.once('close', resolve))
       t.false(fs.existsSync(fileB.capacitor.path), 'Cleanup B.')
+
+      await t.equals(appErrors, 1)
     })
 
     await t.test('Express middleware.', async t => {
@@ -851,7 +856,7 @@ t.test('Aborted request.', async t => {
     }
 
     await t.test('Koa middleware.', async t => {
-      t.plan(5)
+      t.plan(6)
 
       let requestHasBeenReceived
       const requestHasBeenReceivedPromise = new Promise(
@@ -893,7 +898,11 @@ t.test('Aborted request.', async t => {
           finish()
         })
 
+      let appErrors = 0
+      app.on('error', () => appErrors++)
+
       const port = await startServer(t, app)
+
       await sendRequest(port, requestHasBeenReceivedPromise)
       await finished
 
@@ -906,6 +915,8 @@ t.test('Aborted request.', async t => {
       if (!fileB.capacitor.closed)
         await new Promise(resolve => fileB.capacitor.once('close', resolve))
       t.false(fs.existsSync(fileB.capacitor.path), 'Cleanup B.')
+
+      t.equals(appErrors, 1)
     })
 
     await t.test('Express middleware.', async t => {