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_fetchmail.bats

@@ -0,0 +1,51 @@
+load 'test_helper/common'
+
+function setup_file() {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG=$(duplicate_config_for_container .)
+
+  docker run -d --name mail_fetchmail \
+    -v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e ENABLE_FETCHMAIL=1 \
+    --cap-add=NET_ADMIN \
+    -h mail.my-domain.com -t "${NAME}"
+
+  wait_for_finished_setup_in_container mail_fetchmail
+}
+
+function teardown_file() {
+  docker rm -f mail_fetchmail
+}
+
+#
+# processes
+#
+
+@test "checking process: fetchmail (fetchmail server enabled)" {
+  run docker exec mail_fetchmail /bin/bash -c "ps aux --forest | grep -v grep | grep '/usr/bin/fetchmail'"
+  assert_success
+}
+
+#
+# fetchmail
+#
+
+@test "checking fetchmail: gerneral options in fetchmailrc are loaded" {
+  run docker exec mail_fetchmail grep 'set syslog' /etc/fetchmailrc
+  assert_success
+}
+
+@test "checking fetchmail: fetchmail.cf is loaded" {
+  run docker exec mail_fetchmail grep 'pop3.example.com' /etc/fetchmailrc
+  assert_success
+}
+
+#
+# supervisor
+#
+
+@test "checking restart of process: fetchmail" {
+  run docker exec mail_fetchmail /bin/bash -c "pkill fetchmail && sleep 10 && ps aux --forest | grep -v grep | grep '/usr/bin/fetchmail'"
+  assert_success
+}