app-arch/rpm: warn about tests not running under sandbox

Closes: https://bugs.gentoo.org/657500
Package-Manager: Portage-2.3.40, Repoman-2.3.9

app-arch/rpm/rpm-4.14.1.ebuild

@@ -108,6 +108,12 @@ src_install() {
 }
 
 src_test() {
+	# Known to fail with FEATURES=usersandbox (bug #657500):
+	if has usersandbox $FEATURES ; then
+		ewarn "You are emerging ${P} with 'usersandbox' enabled." \
+			"Expect some test failures or emerge with 'FEATURES=-usersandbox'!"
+	fi
+
 	emake check
 }