Move `assert_nothing_raised` method to Rails Specific Assertions

section [ci skip]

- Method `assert_nothing_raised` is Rails' own custom assertion method and
not a part of Minitest. So move it from Minitest assertions section to Rails
Specific Assertions section.

guides/source/testing.md

@@ -312,7 +312,6 @@ specify to make your test failure messages clearer.
 | `assert_not_in_delta( expected, actual, [delta], [msg] )`        | Ensures that the numbers `expected` and `actual` are not within `delta` of each other.|
 | `assert_throws( symbol, [msg] ) { block }`                       | Ensures that the given block throws the symbol.|
 | `assert_raises( exception1, exception2, ... ) { block }`         | Ensures that the given block raises one of the given exceptions.|
-| `assert_nothing_raised { block }`                                | Ensures that the given block doesn't raise any exceptions.|
 | `assert_instance_of( class, obj, [msg] )`                        | Ensures that `obj` is an instance of `class`.|
 | `assert_not_instance_of( class, obj, [msg] )`                    | Ensures that `obj` is not an instance of `class`.|
 | `assert_kind_of( class, obj, [msg] )`                            | Ensures that `obj` is an instance of `class` or is descending from it.|
@@ -343,6 +342,7 @@ Rails adds some custom assertions of its own to the `minitest` framework:
 | --------------------------------------------------------------------------------- | ------- |
 | [`assert_difference(expressions, difference = 1, message = nil) {...}`](http://api.rubyonrails.org/classes/ActiveSupport/Testing/Assertions.html#method-i-assert_difference) | Test numeric difference between the return value of an expression as a result of what is evaluated in the yielded block.|
 | [`assert_no_difference(expressions, message = nil, &block)`](http://api.rubyonrails.org/classes/ActiveSupport/Testing/Assertions.html#method-i-assert_no_difference) | Asserts that the numeric result of evaluating an expression is not changed before and after invoking the passed in block.|
+| [`assert_nothing_raised { block }`](http://api.rubyonrails.org/classes/ActiveSupport/TestCase.html#method-i-assert_nothing_raised) | Ensures that the given block doesn't raise any exceptions.|
 | [`assert_recognizes(expected_options, path, extras={}, message=nil)`](http://api.rubyonrails.org/classes/ActionDispatch/Assertions/RoutingAssertions.html#method-i-assert_recognizes) | Asserts that the routing of the given path was handled correctly and that the parsed options (given in the expected_options hash) match path. Basically, it asserts that Rails recognizes the route given by expected_options.|
 | [`assert_generates(expected_path, options, defaults={}, extras = {}, message=nil)`](http://api.rubyonrails.org/classes/ActionDispatch/Assertions/RoutingAssertions.html#method-i-assert_generates) | Asserts that the provided options can be used to generate the provided path. This is the inverse of assert_recognizes. The extras parameter is used to tell the request the names and values of additional request parameters that would be in a query string. The message parameter allows you to specify a custom error message for assertion failures.|
 | [`assert_response(type, message = nil)`](http://api.rubyonrails.org/classes/ActionDispatch/Assertions/ResponseAssertions.html#method-i-assert_response) | Asserts that the response comes with a specific status code. You can specify `:success` to indicate 200-299, `:redirect` to indicate 300-399, `:missing` to indicate 404, or `:error` to match the 500-599 range. You can also pass an explicit status number or its symbolic equivalent. For more information, see [full list of status codes](http://rubydoc.info/github/rack/rack/master/Rack/Utils#HTTP_STATUS_CODES-constant) and how their [mapping](http://rubydoc.info/github/rack/rack/master/Rack/Utils#SYMBOL_TO_STATUS_CODE-constant) works.|