Fix state sharing bug in .dependency(_:_:) trait

[TimHendricksenDD]

When the .dependency(_:_:) function is used with a reference type to override the dependency for a test suite, tests can inadvertently share state because the same instance is used for each test:

@Suite(.serialized, .dependency(\.classClient, ClassClient()))
struct Example {
  @Test
  func test1() {
    @Dependency(\.classClient) var client
    client.count += 1
    #expect(client.count == 1)
  }

  @Test
  func test2() {
    @Dependency(\.classClient) var client
    client.count += 1
    #expect(client.count == 1) // 🛑 Expectation failed: (client.count → 2) == 1
  }
}

To fix this, this PR changes the value argument to be an @autoclosure so that a new instance can be created for each test. Let me know if there's an alternate solution that would be better!

[stephencelis]

Good catch, thanks!