WL#9541: Move MTR documentation to Doxygen

Doxygen is a tool used for generating documents. The documentation can be written 
within the code or can be maintained in a separate file within the source code and 
thus easy to keep it up to date.

This WL will move the MTR(mysql-test-run) documentation from current 
infrastructure to Doxygen. This will be much easier to maintain for the QA team as 
well.

Functional requirements:
========================
F-1 : Current MTR documentation dev.mysql.com/doc/mysqltest/2.0/en/ should be 
moved to doxygen. The new MTR documentation should be part of existing MySQL 
source code documentation(https://dev.mysql.com/doc/dev/mysql-server/latest/).

Non-functional requirements:
============================
None

I-1: New File

     mysql-test-run.dox - file contatining MTR documentation

I-2: No new syntax

I-3: No new command

I-4: No new tools

I-5: No impact on existing functionality

This WL will move the current MTR 
documentation(https://dev.mysql.com/doc/mysqltest/2.0/en/) to Doxygen. The MTR 
documentation file(mysql-test-run.dox) will be part of MySQL source code and will 
be present under '$SOURCE_DIR/mysql-test' directory'.

Files :
=======
sql/mysqld.cc
    A new section/entry called "Testing Tools" is added to the TOC of the existing 
    MySQL source code documentation main page. New section added will have a     
    subsection called "The MySQL Test Framework, Version 2.0" for MTR 
    documentation.

mysql-test/mysql-test-run.dox
    MTR documentation file

Doxyfile
    - "mysql-test/mysql-test-run.dox" is added to the INPUT tag
    - "mysql-test" is removed from the EXCLUDE tag