Add GHA job step to run template unit tests

[leotm]

Summary:

The added CI check will pass once

• Add @react-native/jest-preset to devDependencies #210

is merged

a CI check is a better approach than e.g. adding a root-level test

// scripts\__tests__\templateYarnTest-test.js
const { execSync } = require('node:child_process')
describe('template', () => {
    it ('run test script', () =>{
        execSync('npm test', { cwd: 'template'}) // bad
    })
})

Resolve: #225

NB: should also be running yarn instead of npm, i'll refactor the workflow after this is merged

Changelog:

[General][Added] - Add GHA job step to run template unit tests

Test Plan:

CI