add plugins test

Author: davidmunechika

packages/driver/cypress/integration/commands/task_spec.js

@@ -210,7 +210,7 @@ describe('src/cy/commands/task', () => {
           expect(lastLog.get('error')).to.eq(err)
           expect(lastLog.get('state')).to.eq('failed')
 
-          expect(err.message).to.eq(`\`cy.task('bar')\` failed with the following error:\n\nThe task 'bar' was not handled in the plugins file. The following tasks are registered: return:arg, arg:is:undefined, wait, create:long:file\n\nFix this in your plugins file here:\n${Cypress.config('pluginsFile')}`)
+          expect(err.message).to.eq(`\`cy.task('bar')\` failed with the following error:\n\nThe task 'bar' was not handled in the plugins file. The following tasks are registered: return:arg, cypress:env, arg:is:undefined, wait, create:long:file\n\nFix this in your plugins file here:\n${Cypress.config('pluginsFile')}`)
 
           done()
         })

packages/driver/cypress/integration/issues/18981_spec.js

@@ -0,0 +1,5 @@
+describe('CYPRESS env var', () => {
+  it('checks that the CYPRESS env var exists in the plugin file', () => {
+    cy.task('cypress:env').should('eq', 'true')
+  })
+})

packages/driver/cypress/plugins/index.js

@@ -38,6 +38,9 @@ module.exports = (on) => {
     'return:arg' (arg) {
       return arg
     },
+    'cypress:env' () {
+      return process.env['CYPRESS']
+    },
     'arg:is:undefined' (arg) {
       if (arg === undefined) {
         return 'arg was undefined'