The HTTP_HOST header is a request header, not a response header. This means that it's sent by the client to the server to specify the domain name the client wants to access. It's not included in the server's response back to the client, which is why you're getting the error message Header [HTTP_HOST] not present on response.
In your test, you're trying to assert that the HTTP_HOST header is present in the response, which is incorrect. If you want to test that the correct domain name was accessed, you should check the url or path of the response, not the headers.
Here's an example of how you might do this:
it('shows the tenant home page')
->get('https://tenant.test', [
'HTTP_HOST' => 'tenant.test',
])
->assertOk()
->assertPathIs('tenant.test');
This will check that the path of the response is tenant.test, which should be true if the correct domain name was accessed.