Merge: nitunit: some fixes and improvements for the `before_all`, `after_all` annotations

This PR does three things:
* fixes the importation of `before_all` and `after_all` methods
* allows use of `before_all` and `after_all` methods within classes
* changes the test execution order for imported test suites

### Fix the importation of `before_all` and `after_all` methods

First of all, this PR fixes the behavior of nitunit with multiple test_suite importation.

Be two modules `a` and `b`:

~~~nit
module a is test

class TestA
    test

    # some test cases
end

fun setup is before_all do # important things
~~~

~~~nit
module b is test
import module a

class TestB
     super TestA
     test
end
~~~

In this case, because `b` does not introduce any `before_all` method, the method from `a` was not executed.
This is fixed now.

### Class-level `before_all` and `after_all` methods

`before_all` and `after_all` can now be used inside a class definition to indicate methods that must be executed before / after all methods inside the class:

~~~nit
class TestC
    test

    fun setup is before_all do # something before all test cases of `TestC`

    # some test cases
end
~~~

### Tests execution order

Methods with `before*` and `after*` annotations are linearized and called in different ways.

* `before*` methods are called from the least specific to the most specific
* `after*` methods are called from the most specific to the least specific

~~~nit
module test_bdd_connector

import bdd_connector

# Testing the bdd_connector
class TestConnector
	test
	# test cases using a server
end

# Method executed before testing the module
fun setup_db is before_all do
	# start server before all test cases
end

# Method executed after testing the module
fun teardown_db is after_all do
	# stop server after all test cases
end
~~~

When dealing with multiple test suites, niunit allows you to import other test suites to factorize your tests:

~~~nit
module test_bdd_users

import test_bdd_connector

# Testing the user table
class TestUsersTable
	test
	# test cases using the db server from `test_bdd_connector`
end

fun setup_table is before_all do
	# create user table
end

fun teardown_table is after_all do
	# drop user table
end
~~~

In the previous example, the execution order would be:

1. `test_bdd_connector::setup_db`
2. `test_bdd_users::setup_table`
3. `all test cases from test_bdd_users`
4. `test_bdd_users::teardown_table`
5. `test_bdd_connector::teardown_db`

Pull-Request: #2572