sys-devel/gdb has: src_test() { nonfatal emake check || ewarn "tests failed" } That's not OK to just ewarn out of failed tests and continue with installation. We have FEATURES=test-fail-continue for that kind of behavior. Please remove the nonfatal || ewarn bits.
gdb-8.3+ fails merge on failing tests: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=0f30b1535e3f79e8a36f678400d78c8f61776746