Configuration files in system tests which require some variables (e.g.
port numbers) filled in during test setup, can now use jinja2 templates
when `jinja2` python package is available.
Any `*.j2` file found within the system test directory will be
automatically rendered with the environment variables into a file
without the `.j2` extension by the pytest runner. E.g.
`ns1/named.conf.j2` will become `ns1/named.conf` during test setup. To
avoid automatic rendering, use `.j2.manual` extension and render the
files manually at test time.
New `templates` pytest fixture has been added. Its `render()` function
can be used to render a template with custom test variables. This can be
useful to fill in different config options during the test. With
advanced jinja2 template syntax, it can also be used to include/omit
entire sections of the config file rather than using `named1.conf.in`,
`named2.conf.in` etc.
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