ci: add testing for mksnapshot (#15627)

* ci: add testing for mksnapshot

spec/fixtures/snapshot-items-available.js

@@ -0,0 +1,20 @@
+// Verifies that objects contained in custom snapshot are accessible in Electron.
+
+const { app } = require('electron')
+
+app.once('ready', () => {
+  try {
+    const testValue = f() // eslint-disable-line no-undef
+    if (testValue === 86) {
+      console.log('ok test snapshot successfully loaded.')
+      app.exit(0)
+    } else {
+      console.log('not ok test snapshot could not be successfully loaded.')
+      app.exit(1)
+    }
+    return
+  } catch (ex) {
+    console.log('Error running custom snapshot', ex)
+    app.exit(1)
+  }
+})

spec/fixtures/testsnap.js

@@ -0,0 +1,3 @@
+// taken from https://chromium.googlesource.com/v8/v8.git/+/HEAD/test/cctest/test-serialize.cc#1127
+function f () { return g() * 2 } // eslint-disable-line no-unused-vars
+function g () { return 43 }