test(file-checker): add test for exercise

sh/tests/file-checker_test.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+IFS='
+'
+
+script_dirS=$(cd -P "$(dirname "$BASH_SOURCE")" &>/dev/null && pwd)
+
+submitted=$(bash "$script_dirS"/student/plus.sh $1)
+expected=$(bash "$script_dirS"/solutions/plus.sh $1)
+
+# check that test command was not used
+grep -q "test" $1
+
+if [ $? -eq 0 ]
+then
+    echo "The 'test' command is not allowed in this exercise"
+    exit 1
+fi
+
+diff <(echo "$submitted") <(echo "$expected")