CondCore/IOVService: fix include (testPayloadObj.h) to relocatable path

testPayloadObj.h -> CondCore/IOVService/test/testPayloadObj.h

$CMSSW_RELEASE_BASE/src/CondCore/IOVService/test is not part of
$ROOT_INCLUDE_PATH. Re-adjust it to allow ROOT6 run-time to
find it.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>