add comment about fd3

Signed-off-by: Nikita Pivkin <nikita.pivkin@smartforce.io>

test/test.bats

@@ -6,6 +6,8 @@ bats_load_library bats-file
 
 setup_file() {
   setup_trivy_env
+  # bats provides fd 3 for printing directly to the terminal
+  # see https://bats-core.readthedocs.io/en/stable/writing-tests.html#printing-to-the-terminal
   docker pull knqyf263/vuln-image:1.2.3 1>&3 2>&3
   trivy image --download-db-only 1>&3 2>&3
 }