It can be used to test the mysqlc connector whenever there is an
external mysql or mariadb server running.

The test runs only when CONNECTIVITY_TEST_MYSQL_DRIVER environment variable
is set. This variable should contain the URL of the database, including
the port number and the host name.

The URL should also contain a user name password pair which can be used
to connect to the external database. The URL format is the following:

[user]/[passwd]@sdbc:mysql:mysqlc:[host]:[port]/[db_name]

README is updated and contains detailed information about the test.

Change-Id: I1bbc9369ff193a29c06de63a0f6cc975877c8da3
Reviewed-on: https://gerrit.libreoffice.org/62171
Tested-by: Jenkins
Reviewed-by: Tamás Bunth <btomi96@gmail.com>