TestSetUpHelper should use DB_ Postgres instance if available

https://github.com/folio-org/folio-service-tools TestSetUpHelper should use DB_ Postgres instance if available.

PostgresClient constructor, by default, starts Embedded Postgres since RMB v26.
This means that if you have unit tests that do not start Embedded Postgres
explicitly, you might need to terminate Embedded Postgres by calling
PostgresClient.stopEmbeddedPostgres:
https://github.com/folio-org/raml-module-builder/blob/master/doc/upgrading.md#version-26

Purpose

Starting embedded postgres takes very long under Windows.
Many developers want to run the tests against a different postgres instance, the connection parameters are provided in the DB_ enviroment variables:
https://github.com/folio-org/raml-module-builder#environment-variables
The current code ignores the DB_ variables but it shouldn't.

Approach

Let RMB start postgres if needed.