Create a common test package for opentelemetry-lib remappers

[gizas]

Currently opentelemetry-lib includes no specific testing package. As we plan to merge more datastreams rather than hostmetrics (eg. kubernetes one), there are common fucntions that are being used in testing.

See eg here: https://github.com/elastic/opentelemetry-lib/pull/20/files#diff-973a344d29ac8e238d1e425d5a5ee843244cbe07fd85a3c1598704af3967a6d0

Plan of this issue is to move the common testing functions in a seperate testing package

[gizas]

Dont merge until #20 (comment) is merged

cc @lahsivjar