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_with_relays.bats

@@ -0,0 +1,81 @@
+load 'test_helper/common'
+
+function setup_file() {
+  # We use a temporary config directory since we'll be dynamically editing
+  # it with setup.sh.
+  tmp_confdir=$(mktemp -d /tmp/docker-mailserver-config-relay-hosts-XXXXX)
+  cp -a test/config/relay-hosts/* "${tmp_confdir}/"
+
+  docker run -d --name mail_with_relays \
+    -v "${tmp_confdir}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e RELAY_HOST=default.relay.com \
+    -e RELAY_PORT=2525 \
+    -e RELAY_USER=smtp_user \
+    -e RELAY_PASSWORD=smtp_password \
+    -e PERMIT_DOCKER=host \
+    -h mail.my-domain.com -t "${NAME}"
+
+  wait_for_finished_setup_in_container mail_with_relays
+}
+
+function teardown_file() {
+  docker rm -f mail_with_relays
+  rm -rf "${tmp_confdir}"
+}
+
+@test "checking relay hosts: default mapping is added from env vars" {
+  run docker exec mail_with_relays grep -e domainone.tld /etc/postfix/relayhost_map
+  assert_output -e '^@domainone.tld[[:space:]]+\[default.relay.com\]:2525$'
+}
+
+@test "checking relay hosts: default mapping is added from env vars for virtual user entry" {
+  run docker exec mail_with_relays grep -e domain1.tld /etc/postfix/relayhost_map
+  assert_output -e '^@domain1.tld[[:space:]]+\[default.relay.com\]:2525$'
+}
+
+@test "checking relay hosts: default mapping is added from env vars for new user entry" {
+  run docker exec mail_with_relays grep -e domainzero.tld /etc/postfix/relayhost_map
+  assert_output ''
+
+  run ./setup.sh -c mail_with_relays email add user0@domainzero.tld password123
+  run_until_success_or_timeout 10 docker exec mail_with_relays grep -e domainzero.tld /etc/postfix/relayhost_map
+  assert_output -e '^@domainzero.tld[[:space:]]+\[default.relay.com\]:2525$'
+}
+
+@test "checking relay hosts: default mapping is added from env vars for new virtual user entry" {
+  run docker exec mail_with_relays grep -e domain2.tld /etc/postfix/relayhost_map
+  assert_output ''
+
+  run ./setup.sh -c mail_with_relays alias add user2@domain2.tld user2@domaintwo.tld
+  run_until_success_or_timeout 10 docker exec mail_with_relays grep -e domain2.tld /etc/postfix/relayhost_map
+  assert_output -e '^@domain2.tld[[:space:]]+\[default.relay.com\]:2525$'
+}
+
+@test "checking relay hosts: custom mapping is added from file" {
+  run docker exec mail_with_relays grep -e domaintwo.tld /etc/postfix/relayhost_map
+  assert_output -e '^@domaintwo.tld[[:space:]]+\[other.relay.com\]:587$'
+}
+
+@test "checking relay hosts: ignored domain is not added" {
+  run docker exec mail_with_relays grep -e domainthree.tld /etc/postfix/relayhost_map
+  assert_failure 1
+  assert_output ''
+}
+
+@test "checking relay hosts: sasl_passwd exists" {
+  run docker exec mail_with_relays [ -f /etc/postfix/sasl_passwd ]
+  assert_success
+}
+
+@test "checking relay hosts: auth entry is added" {
+  run docker exec mail_with_relays /bin/sh -c 'cat /etc/postfix/sasl_passwd | grep -e "^@domaintwo.tld\s\+smtp_user_2:smtp_password_2" | wc -l'
+  assert_success
+  assert_output 1
+}
+
+@test "checking relay hosts: default auth entry is added" {
+  run docker exec mail_with_relays /bin/sh -c 'cat /etc/postfix/sasl_passwd | grep -e "^\[default.relay.com\]:2525\s\+smtp_user:smtp_password" | wc -l'
+  assert_success
+  assert_output 1
+}