Enable testSuiteAccessControl rule

[calda]

Summary

This PR proposes a new test suite access control rule. In a test suite, test cases should be internal and other properties / helpers should be private.

Reasoning

Test suites and test cases don't need to be public to be picked up by XCTest / Swift Testing.

Helpers and stored properties should be private since they are not accessed outside of the test case.

import Testing

// WRONG
struct SpaceshipTests {
  let spaceship = Spaceship()

  func launchSpaceship() {
    spaceship.launch()
  }

  @Test
  func spaceshipCanLaunch() {
    launchSpaceship()
    #expect(spaceship.hasLaunched)
  }
}

// RIGHT
struct SpaceshipTests {

  // MARK: Internal

  @Test
  func spaceshipCanLaunch() {
    launchSpaceship()
    #expect(spaceship.hasLaunched)
  }

  // MARK: Private

  private let spaceship = Spaceship()

  private func launchSpaceship() {
    spaceship.launch()
  }

}