Fix flaky sandbox tests

Windows apparently has issues running opening two windows at once which is causing flakiness on the mixed sandbox tests.

spec/fixtures/api/mixed-sandbox-app/main.js

@@ -10,27 +10,32 @@ if (!process.argv.includes('--enable-mixed-sandbox')) {
   app.enableMixedSandbox()
 }
 
-let sandboxWindow
-let noSandboxWindow
+let currentWindowSandboxed = false
 
 app.once('ready', () => {
-  sandboxWindow = new BrowserWindow({
-    show: false,
-    webPreferences: {
-      preload: path.join(__dirname, 'electron-app-mixed-sandbox-preload.js'),
-      sandbox: true
-    }
-  })
-  sandboxWindow.loadURL('about:blank')
-
-  noSandboxWindow = new BrowserWindow({
-    show: false,
-    webPreferences: {
-      preload: path.join(__dirname, 'electron-app-mixed-sandbox-preload.js'),
-      sandbox: false
-    }
-  })
-  noSandboxWindow.loadURL('about:blank')
+  function testWindow (isSandboxed, callback) {
+    currentWindowSandboxed = isSandboxed
+    let currentWindow = new BrowserWindow({
+      show: false,
+      webPreferences: {
+        preload: path.join(__dirname, 'electron-app-mixed-sandbox-preload.js'),
+        sandbox: isSandboxed
+      }
+    })
+    currentWindow.loadURL('about:blank')
+    currentWindow.webContents.once('devtools-opened', () => {
+      if (isSandboxed) {
+        argv.sandboxDevtools = true
+      } else {
+        argv.noSandboxDevtools = true
+      }
+      if (callback) {
+        callback()
+      }
+      finish()
+    })
+    currentWindow.webContents.openDevTools()
+  }
 
   const argv = {
     sandbox: null,
@@ -41,6 +46,10 @@ app.once('ready', () => {
 
   let connected = false
 
+  testWindow(true, () => {
+    testWindow()
+  })
+
   function finish () {
     if (connected && argv.sandbox != null && argv.noSandbox != null &&
         argv.noSandboxDevtools != null && argv.sandboxDevtools != null) {
@@ -57,22 +66,10 @@ app.once('ready', () => {
     finish()
   })
 
-  noSandboxWindow.webContents.once('devtools-opened', () => {
-    argv.noSandboxDevtools = true
-    finish()
-  })
-  noSandboxWindow.webContents.openDevTools()
-
-  sandboxWindow.webContents.once('devtools-opened', () => {
-    argv.sandboxDevtools = true
-    finish()
-  })
-  sandboxWindow.webContents.openDevTools()
-
   ipcMain.on('argv', (event, value) => {
-    if (event.sender === sandboxWindow.webContents) {
+    if (currentWindowSandboxed) {
       argv.sandbox = value
-    } else if (event.sender === noSandboxWindow.webContents) {
+    } else {
       argv.noSandbox = value
     }
     finish()