Improve tests for EMF exporter

[anuraaga]

Is your feature request related to a problem? Please describe.
Currently, we have tests like this one

https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/master/exporter/awsemfexporter/emf_exporter_test.go#L114

All it does is verify that a credentials error is raised when calling this code. Is this the intended test case? If so, the name of the test needs to be renamed.

@shaochengwang Can you confirm this?

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