Decide how to package/organize test-helper sources

[magicDGS]

After #14, the test-module (org.htsjdk.test) is included into the main artifact for the core-module because there are problems with circular dependencies (see discussion in #14 (comment) and #14 (comment)).

We should separate the artifacts to provide everything as an independent one (e.g., htsjdk-core.jar, htsjdk-testutils.jar, htsjdk-cram.jar), but also keeping the uber-jar with all the project (htsjdk-all.jar) separated from the test-utilities, which are dependent on some frameworks exclusively for testing (TestNG, Mockito, etc).

Several ideas to fix this are:

• Generate an artifact from the core-module tests, which contains the test-utilities (but also the tests), and use it as a dependent artifact for the rest of the modules (also, provide it for downstream projects).
• Create a new test-module containing in the sources the test-utilities to distribute (maybe including some files to also attach for downstream testing) and in the test-sources testing also the core-module. Thus, unit-tests for a concrete class in core would live in the test-module src/test folder. This will require some guidelines to indicate that the tests should be moved there.
• Something similar to GATK (Splitting out gatk-testUtils as a separate artifact broadinstitute/gatk#5112), where the sources for the test-utils live in a different source set that is build in a different artifact.