CI: prepare for enabling shellcheck warnings and info

- Assume /bin/sh has dash's features and ignore corresponding false positives

- Exclude parser/tst, tests and utils/test directories: they have tons
  of shellcheck violations but they don't *directly* impact our users.
  Let's first focus on code that runs on production systems.

- Exclude rc.apparmor.slackware: I don't know anything about Slackware's
  /bin/sh.

.shellcheckrc

@@ -0,0 +1,10 @@
+# Don't follow source'd scripts
+disable=SC1090
+disable=SC1091
+
+# dash supports 'local'
+disable=SC2039
+disable=SC3043
+
+# dash supports 'echo -n'
+disable=SC3037

tests/bin/shellcheck-tree

@@ -8,8 +8,10 @@ from pathlib import Path
 
 
 def is_excluded(f):
-    return re.match(r"^([.]git)/",
-                    f) or Path(f).is_dir()
+    return (re.match(
+        r"^([.]git/|parser/tst/|tests/|utils/test/|parser/rc[.]apparmor[.]slackware)",
+        f,
+    ) or Path(f).is_dir())
 
 
 def mimetype(f):