Fix custom log level manipulation and isolate logger unit tests

[ericmustin]

Addressing #681 , a custom logger's log level can be unintentionally changed by Tracer configuration, which I was able to recreate locally.

• Per suggestion this PR adds a check in self.debug_logging assignment for whether the custom logger is an instance of the Datadog::Logger class and is a noop if debug logging is not enabled
• Adds a unit test in test/logger_test.rb for new behavior.
• While I was in there I noticed the logger unit tests could be isolated a bit more, as failed assertions would cause some of the teardown code at the end of each unit test to not get called. Following some of the conventions used in other tests, this PR also adds setup and teardown helpers for logger_test.rb.

Thanks for taking a look! Happy to address any feedback or anything else you think might be relevant here.

[delner]

@ericmustin Thanks for the contribution! Looks good overall. We're in the process of converting those minitests to RSpec, where I think the teardown issue you ran into won't be such a factor.

Going to merge and release this with 0.21.0. 🎉