Add Health Checks

[bradp]

Proposed changes

Implements a variety of Health Checks to the Site Health page.

Jira tickets: PRESS7-108, PRESS7-109, PRESS7-110, PRESS7-111, PRESS7-118, PRESS7-112, PRESS7-113, PRESS7-121, PRESS7-107, PRESS7-119, PRESS7-120, PRESS7-114, PRESS7-115, PRESS7-116, PRESS7-117.

Type of Change

• New feature (non-breaking change which adds functionality)

Screenshot

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

This PR adds HealthCheckManager.php which is a helper class to set up and add health checks to the Site Health page. This is used in HealthChecks.php to add the health checks to the Site Health page.

This makes it so adding a new health check is as simple as:

$manager->addHealthCheck( [
  'id'    => 'example-health-check',
  'title' => __( 'Example Health Check', 'newfold-performance-module' ),
  'pass'  => __( 'Example health check passed', 'newfold-performance-module' ),
  'fail'  => __( 'Example health check failed', 'newfold-performance-module' ),
  'text'  => __( 'This is some text to explain the health check to the user.', 'newfold-performance-module' ),
  'test'  => function () {
    // Example test for a constant being defined and having a specific value.
    return ( defined( 'EXAMPLE_CONSTANT' ) && EXAMPLE_CONSTANT === 'example-value' );
  },
] );

This PR also adds Health Checks that are commented out for the following PRs

• Enhance/press7 60 jetpack boost #25
• link prefetch new feature #26
• Image Optimization: Add Lazy Loading Support #32