MDL-38785 behat: New tests

According to MDLQA-943, users can
add comments via a comments block

blocks/comments/tests/behat/add_comment.feature

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+@core_comment
+Feature: Add a comment to the comments block
+  In order to comment on a conversation or a topic
+  As a moodle user
+  In need to add comments to courses
+
+  Background:
+    Given the following "courses" exists:
+      | fullname | shortname | category |
+      | Course 1 | C1 | 0 |
+    And the following "users" exists:
+      | username | firstname | lastname | email |
+      | teacher1 | Teacher | Frist | teacher1@asd.com |
+      | student1 | Student | First | student1@asd.com |
+    And the following "course enrolments" exists:
+      | user | course | role |
+      | teacher1 | C1 | editingteacher |
+      | student1 | C1 | student |
+    And I log in as "teacher1"
+    And I follow "Course 1"
+    And I turn editing mode on
+    And I add the "Comments" block
+    And I log out
+    And I log in as "student1"
+    And I follow "Course 1"
+
+  @javascript
+  Scenario: Add a comment with Javascript enabled
+    When I add "I'm a comment from student1" comment to comments block
+    Then I should see "I'm a comment from student1"
+
+  Scenario: Add a comment with Javascript disabled
+    When I follow "Show comments"
+    And I add "I'm a comment from student1" comment to comments block
+    Then I should see "I'm a comment from student1"

blocks/comments/tests/behat/delete_comment.feature

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+@core_comment
+Feature: Delete comment block messages
+  In order to refine comment block's contents
+  As a moodle teacher
+  In need to delete comments from courses
+
+  @javascript
+  Scenario: Delete comments with Javascript enabled
+    Given the following "courses" exists:
+      | fullname | shortname | category |
+      | Course 1 | C1 | 0 |
+    And the following "users" exists:
+      | username | firstname | lastname | email |
+      | teacher1 | Teacher | First | teacher1@asd.com |
+      | student1 | Student | First | student1@asd.com |
+    And the following "course enrolments" exists:
+      | user | course | role |
+      | teacher1 | C1 | editingteacher |
+      | student1 | C1 | student |
+    And I log in as "teacher1"
+    And I follow "Course 1"
+    And I turn editing mode on
+    And I add the "Comments" block
+    And I log out
+    And I log in as "student1"
+    And I follow "Course 1"
+    And I add "Comment from student1" comment to comments block
+    And I log out
+    And I log in as "teacher1"
+    And I follow "Course 1"
+    And I add "Comment from teacher1" comment to comments block
+    When I delete "Comment from student1" comment from comments block
+    Then I should not see "Comment from student1"
+    And I delete "Comment from teacher1" comment from comments block
+    And I should not see "Comment from teacher1"