Add a assertDatabaseHasCount assertion to the available database assertions

[yugarinn]

I sometimes find myself with the need of asserting that a table in the database contains a given number of records matching a given query constraints.

This new assertion would function similarly to the existing assertDatabaseHas method but would include an additional parameter for the expected number of rows.

To illustrate, here are two practical use cases for this assertion:

public function test_it_does_not_allow_to_create_users_with_already_taken_emails(): void
{
    $email = '[email protected]';
    $user = User::factory()->make(['email' => $email]);

    $response = $this->json('POST', "/users", ['email' => $email]);

    $response->assertStatus(422);
    $this->assertDatabaseHasCount('users', 1, ['email' => $email]);
}

public function test_it_successfully_duplicates_a_task(): void
{
    $taskTitle = 'Locate the Ark of the Covenant.';
    $task = Task::factory()->create(['title' => $taskTitle]);

    $response = $this->json('POST', "/tasks/{$task->id}/duplicate");

    $response->assertStatus(200);
    $this->assertDatabaseHasCount('tasks', 2, ['title' => $taskTitle]);
}

I looked at the InteractsWithDatabase trait and I think this addition would be pretty straightforward. If this is something the community and the core team find valuable I am willing and available to implement it.