As partially planned during do-test implementation and suggested by more people later, enable TEST option by default if DEVELOPER is defined.
I'll remind that TEST option is supposed to be used when a port needs extra dependencies and/or compile options on BUILD phase to enable tests (so TEST_DEPENDS won't work). Now enable it when DEVELOPER is defined so all interested parties (committers/maintainers who set DEVELOPER and Q/A poudriere builds) have tests enabled.