Set up Combination Testing for SDK 3 - Server 5.5, 6.0, 6.5

Across 5.5, 6.0 and 6.5 server platforms, set up combination testing to execute regular feature tests across all of these platforms per our build and test plan.

Note that the plan is to support 6.0 and 6.5 for certain, extending back to 5.5 if tests look good and no significant issues are encountered (despite 5.5 being EOL, but not EOSL in April 2020).

Since it should be simple to do, add 5.0 just to characterize for known issues to document if possible. No support of 5.0 is planned.