Only run live MySQL tests on correct MySQL version

[preaction]

http://www.cpantesters.org/cpan/report/ada1c782-844e-11e7-a7c3-e1b61dee0349

We need a MySQL version that has the JSON field, so 5.7 or above. Without this, the tests will fail since it can't create the schema.

We need to detect the MySQL version (Test::mysqld should have a function for this), and skip the relevant tests if it's not a version we can support.

[lindleyw]

Also note that MariaDB as of at least 10.2.11 although JSON is included, does not yet support the ->> operator, also causing tests to fail. Mildly irksome is that Debian 9 only includes MariaDB as the only option by default..