add tests for backends

[willkg]

We currently have no tests for markus.configure or the backends. We should probably add at least some basic ones.

• test markus.configure with zero, one, and multiple backends does the right thing
• test markus.configure with backend options passes the options in
• test markus.configure with backend filters passes the filters in
• test that filters work on the backends

Probably some other things, too.

[jwhitlock]

I'd add 'raise / log error on unknown configuration option'. We got bit by the statsd backend using statsd_prefix, but the datadog backend using statsd_namespace.