do not print warnings when listing tests

[igagis]

Steps:
. build all: make
. list tests from basic test app: tests/basic/out/rel/tests --list-tests

Result:
two warnings:

warning: some test set for suite 'empty_suite' is empty
warning: suite 'empty_suite' is empty

are printed before the tests list

Expected result:
when listing tests, there should be no warnings printed