Issue:
------
New test case mysql_embedded.test was failing on pb2.

Description:
------------
To run this test case executable libmysqld/examples/mysql_embedded is required.
But as per /libmysqld/examples/cmake_install.cmake this executable doesn't get
copied to <install_dir> when mysql is installed at <install_dir>.That is the
reason it was passing in my local branch and failed on pb2 when pushed.

Solution;
---------
Added code in mysql-test-run.pl, which will try to see if this file exists.If
It doesn't exist, test case will be skipped with a skip message. New code in
mysql-test-run.pl looks only for directory libmysqld/examples/mysql_embedded
because this is the only place where this file could/does exist.

mysql-test/mysql-test-run.pl:
  Added new variable for mysql_embedded executable.
mysql-test/t/disabled.def:
  enabled mysql_embedded.test which was disabled earlier.
mysql-test/t/mysql_embedded.test:
  Modified test case to first verify if mysql_embedded executable exists. If
  it does not, skip the test.