[pkg/stanza/fileconsumer] Decouple test setup code (open-telemetry#12771

)

This package has some test utility functions that create a "default"
config that is different than the actual default config. In order for
individual test cases to create their desired configs, they typically
ask for this alternate "default" config and mutate it. The problem is
that setting up a config require reasoning about two layers of defaults,
which is error prone.

This PR simplifies the shared portion of setup code, and pushing a
small amount of very easy setup back into the individual test cases.
This results in clearer tests.