docs: Add new local dependency (`file`) for running tests (#3856)

docs/content/contributing/tests.md

@@ -51,7 +51,7 @@ Parallel tests are further partitioned into smaller sets. If your system has the
 To run the test suite, you will need to:
 
 1. [Install Docker][get-docker]
-2. Install `jq` and (GNU) `parallel` (under Ubuntu, use `sudo apt-get -y install jq parallel`)
+2. Install `jq` , (GNU) `parallel` and `file` (under Ubuntu, use `sudo apt-get -y install jq parallel file`)
 3. Execute `git submodule update --init --recursive` if you haven't already initialized the git submodules
 
 ### Executing Test(s)

target/postfix/main.cf

@@ -119,6 +119,6 @@ smtp_header_checks = pcre:/etc/postfix/maps/sender_header_filter.pcre
 
 # The default compatibility_level is 0 - which retains legacy settings defaults:
 # http://www.postfix.org/COMPATIBILITY_README.html
-# If backwards-compaitibilty log messages appear, fix them by explicitly adding
+# If backwards-compatibility log messages appear, fix them by explicitly adding
 # the legacy or new default value (alternatively raise the compatibility_level)
 compatibility_level = 3.6