tests(chore): Rename test files to serial and parallel types

- `test_helper.bats` needs more work than this PR provides to be compatible with parallel tests, so must remain as a serial test for now.
- `spam_bounced.bats` had failures as a serial test, but works well converted to a parallel test in a future commit.

test/tests/serial/mail_postfix_inet.bats

@@ -0,0 +1,80 @@
+load 'test_helper/common'
+
+# Test case
+# ---------
+# POSTFIX_INET_PROTOCOLS value is set
+
+@test "checking postfix: inet default" {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG=$(duplicate_config_for_container . )
+
+  docker run -d --name mail_postfix_inet_default \
+    -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -h mail.my-domain.com -t "${NAME}"
+
+  teardown() { docker rm -f mail_postfix_inet_default; }
+
+  wait_for_finished_setup_in_container mail_postfix_inet_default
+
+  run docker exec mail_postfix_inet_default postconf inet_protocols
+  assert_output "inet_protocols = all"
+  assert_success
+}
+
+@test "checking postfix: inet all" {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG=$(duplicate_config_for_container . )
+
+  docker run -d --name mail_postfix_inet_all \
+    -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e POSTFIX_INET_PROTOCOLS=all \
+    -h mail.my-domain.com -t "${NAME}"
+
+  teardown() { docker rm -f mail_postfix_inet_all; }
+
+  wait_for_finished_setup_in_container mail_postfix_inet_all
+
+  run docker exec mail_postfix_inet_all postconf inet_protocols
+  assert_output "inet_protocols = all"
+  assert_success
+}
+
+@test "checking postfix: inet ipv4" {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG=$(duplicate_config_for_container . )
+
+  docker run -d --name mail_postfix_inet_ipv4 \
+    -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e POSTFIX_INET_PROTOCOLS=ipv4 \
+    -h mail.my-domain.com -t "${NAME}"
+
+  teardown() { docker rm -f mail_postfix_inet_ipv4; }
+
+  wait_for_finished_setup_in_container mail_postfix_inet_ipv4
+
+  run docker exec mail_postfix_inet_ipv4 postconf inet_protocols
+  assert_output "inet_protocols = ipv4"
+  assert_success
+}
+
+@test "checking postfix: inet ipv6" {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG=$(duplicate_config_for_container . )
+
+  docker run -d --name mail_postfix_inet_ipv6 \
+    -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e POSTFIX_INET_PROTOCOLS=ipv6 \
+    -h mail.my-domain.com -t "${NAME}"
+
+  teardown() { docker rm -f mail_postfix_inet_ipv6; }
+
+  wait_for_finished_setup_in_container mail_postfix_inet_ipv6
+
+  run docker exec mail_postfix_inet_ipv6 postconf inet_protocols
+  assert_output "inet_protocols = ipv6"
+  assert_success
+}