(testsuite) Add check for awk with POSIX character classes.

test/runLint

@@ -16,7 +16,10 @@ cd $(dirname "$0") ; cd ..
 
 gitgrep "\bawk\b.*-F([[:space:]]|[[:space:]]*[\"'][^\"']{2,})" \
     'awk with -F char or -F ERE, use -Fchar instead (Solaris)'
-    
+
+gitgrep "\bawk\b.*\[:[a-z]*:\]" \
+    'awk with POSIX character class not supported in mawk (Debian/Ubuntu)'
+
 gitgrep '\bsed\b.*\\[?+]' \
     'sed with ? or +, use POSIX BRE instead (\{m,n\})'