tests: Adjusted files not directly related to tests

`tls.bash` helper was adapted to the new helper scripts location. The `setup.bash` helper saw a bugfix (expanding the array properly) and updates the container default config to configure for IPv4 explicitly.

The IPv4 default was added after recent Docker pushes and I saw weird IPv6 related errors in the logs.. now we're sure IPv4 is the default during tests.

Added functionality to check if a process is running:
- This change adds a helper function to check whether a program is running inside a container or not.
- This added the need for a function like `_run_in_container` but allowing for providing an explicit container name.
- Future PRs can use this helper function now to check whether a process is running or not. This was done for the tests of Fail2Ban, but can be used for other tests in the future as well.

---

chore: Restructured BATS flags in `Makefile`

The `Makefile` has seen a bit of a restructuring when it comes to flags:

1. The `MAKEFLAGS` variables is used by `make`, and allows for adding additional flags that can be used within in recursive calls (via `$(MAKE)`) too,  thus DRY approach.
2. The flags for calling BATS were adjusted. `--no-parallelize-within-files` has been added as well to ensure tests  _inside_ a single file are run sequentially.

`dms-test` prefix matching changed to expect a `_` suffix as a delimiter.

---

docs: Add a note regarding output from running tests in parallel

test/helper/setup.bash

@@ -17,14 +17,15 @@ function __initialize_variables() {
     'CONTAINER_NAME'
   )
 
-  for VARIABLE in "${REQUIRED_VARIABLES_FOR_TESTS}"
+  for VARIABLE in "${REQUIRED_VARIABLES_FOR_TESTS[@]}"
   do
     __check_if_set "${VARIABLE}"
   done
 
+  export SETUP_FILE_MARKER TEST_TIMEOUT_IN_SECONDS NUMBER_OF_LOG_LINES
+  SETUP_FILE_MARKER="${BATS_TMPDIR:?}/$(basename "${BATS_TEST_FILENAME:?}").setup_file"
   TEST_TIMEOUT_IN_SECONDS=${TEST_TIMEOUT_IN_SECONDS:-120}
   NUMBER_OF_LOG_LINES=${NUMBER_OF_LOG_LINES:-10}
-  SETUP_FILE_MARKER="${BATS_TMPDIR:?}/$(basename "${BATS_TEST_FILENAME:?}").setup_file"
 }
 
 # -------------------------------------------------------------------
@@ -118,6 +119,8 @@ function common_container_create() {
     --env ENABLE_UPDATE_CHECK=0 \
     --env ENABLE_SPAMASSASSIN=0 \
     --env ENABLE_FAIL2BAN=0 \
+    --env POSTFIX_INET_PROTOCOLS=ipv4 \
+    --env DOVECOT_INET_PROTOCOLS=ipv4 \
     --env LOG_LEVEL=debug \
     "${X_EXTRA_ARGS[@]}" \
     "${IMAGE_NAME}"