Added assert_link_exist function and use that

test/test_helper.bash

@@ -39,3 +39,48 @@ teardown() {
 
   rm -rf "${BASH_IT_TEST_DIR}"
 }
+
+# Fail and display path of the link if it does not exist. Also fails
+# if the path exists, but is not a link.
+# This function is the logical complement of `assert_file_not_exist'.
+# There is no dedicated function for checking that a link does not exist.
+#
+# Globals:
+#   BATSLIB_FILE_PATH_REM
+#   BATSLIB_FILE_PATH_ADD
+# Arguments:
+#   $1 - path
+# Returns:
+#   0 - link exists and is a link
+#   1 - otherwise
+# Outputs:
+#   STDERR - details, on failure
+assert_link_exist() {
+  local -r file="$1"
+  local -r target="$2"
+  if [[ ! -L "$file" ]]; then
+    local -r rem="$BATSLIB_FILE_PATH_REM"
+    local -r add="$BATSLIB_FILE_PATH_ADD"
+    if [[ -e "$file" ]]; then
+      batslib_print_kv_single 4 'path' "${file/$rem/$add}" \
+        | batslib_decorate 'exists, but is not a link' \
+        | fail
+    else
+      batslib_print_kv_single 4 'path' "${file/$rem/$add}" \
+        | batslib_decorate 'link does not exist' \
+        | fail
+    fi
+  else
+    if [ -n "$target" ]; then
+      local link_target=''
+      link_target=$(readlink "$file")
+      if [[ "$link_target" != "$target" ]]; then
+        batslib_print_kv_single_or_multi 8 'path' "${file/$rem/$add}" \
+            'expected' "$target" \
+            'actual'   "$link_target" \
+          | batslib_decorate 'link exists, but does not point to target file' \
+          | fail
+      fi
+    fi
+  fi
+}