spec: Do not assume which process exits first in singleton test

spec/api-app-spec.js

@@ -162,19 +162,23 @@ describe('app module', function () {
       this.timeout(120000)
       const appPath = path.join(__dirname, 'fixtures', 'api', 'singleton')
       // First launch should exit with 0.
-      let secondLaunched = false
+      let exited = 0
       const first = ChildProcess.spawn(remote.process.execPath, [appPath])
       first.once('exit', (code) => {
-        assert.ok(secondLaunched)
         assert.equal(code, 0)
-        done()
+        exited++
+        if (exited === 2) {
+          done()
+        }
       })
       // Second launch should exit with 1.
       const second = ChildProcess.spawn(remote.process.execPath, [appPath])
       second.once('exit', (code) => {
-        assert.ok(!secondLaunched)
         assert.equal(code, 1)
-        secondLaunched = true
+        exited++
+        if (exited === 2) {
+          done()
+        }
       })
     })
   })