PHPUnit tests for is_main_network

[pbearne]

npm run test:php -- -c tests/phpunit/multisite.xml --group ms-site --filter Tests_MultiSite_is_main_network

[desrosj]

Since this is about adding tests and not a general build or test tooling change, I'm reassigning it to the relevant component.

Trac Ticket: Core-59981

## Description:

• This pull request introduces the initial set of unit tests for the is_main_network() function. The tests are designed to verify the function's behavior in both multisite and single-site scenarios.

## Changes Made:

• Added Multisite Tests:

• Class: Tests_Multisite_IsMainNetwork

• Purpose: Tests the behavior of is_main_network() for valid and invalid network IDs within a multisite environment.

• Test Cases:

• Valid network IDs that should return true:
  • null
  • 1 (main network)

• Invalid network IDs that should return false:
  • 2
  • 0
  • Empty string

• Added Single-Site Tests:

• Class: Tests_Functions_IsMainNetwork

• Purpose: Tests the behavior of is_main_network() in a single-site context.

• Test Cases:

• Valid network IDs that should return true:
  • null
  • 1
  • 0
  • Empty string
  • Non-main network ID (2) which is also tested to ensure the function handles it correctly.

## Benefits:

• Coverage for Multisite and Single-Site

• Ensures that the is_main_network() function is thoroughly tested in different contexts.

• Early Detection of Issues

• The introduction of these tests will help catch any bugs or unexpected behavior early in the development process.

## Testing:

• The tests are designed to be run in the WordPress environment using PHPUnit.

• All tests have been verified to pass successfully.

## Commands

### Multisite

npm run test:php -- -c tests/phpunit/multisite.xml --group ms-site --filter Tests_MultiSite_is_main_network

### Single SIte

npm run test:php --filter Tests_Multisite_IsMainNetwork::test_should_return_true_for_valid_network_id