message: Fix unused warnings in filesystem tests
diff: #include <gtest/gtest.h>
  +
  #include "fs.h"
  +#include "unused.h"
  #if !defined(WINDOWS)
  #define PATH_MAX 4096
  @@ -43,7 +45,7 @@ TEST(Filesystem, ResolveDoubleDots)
  TEST(Filesystem, RelativePathEmpty)
  {
  char cwd[PATH_MAX];
  - getcwd(cwd, PATH_MAX);
  + UNUSED(getcwd(cwd, PATH_MAX));
  const std::string path = "";
  const std::string canonical_path = std::string(cwd);
  ASSERT_EQ(canonical_path, mavsdk::fs_canonical(path));
  @@ -52,7 +54,7 @@ TEST(Filesystem, RelativePathEmpty)
  TEST(Filesystem, RelativePathDot)
  {
  char cwd[PATH_MAX];
  - getcwd(cwd, PATH_MAX);
  + UNUSED(getcwd(cwd, PATH_MAX));
  const std::string path = ".";
  const std::string canonical_path = std::string(cwd);
  ASSERT_EQ(canonical_path, mavsdk::fs_canonical(path));
  @@ -61,7 +63,7 @@ TEST(Filesystem, RelativePathDot)
  TEST(Filesystem, RelativePathBare)
  {
  char cwd[PATH_MAX];
  - getcwd(cwd, PATH_MAX);
  + UNUSED(getcwd(cwd, PATH_MAX));
  const std::string path = "src/mavsdk/core/fs_test.cpp";
  const std::string canonical_path = std::string(cwd) + mavsdk::path_separator + path;
  ASSERT_EQ(canonical_path, mavsdk::fs_canonical(path));
  @@ -70,7 +72,7 @@ TEST(Filesystem, RelativePathBare)
  TEST(Filesystem, RelativePathDotSlash)
  {
  char cwd[PATH_MAX];
  - getcwd(cwd, PATH_MAX);
  + UNUSED(getcwd(cwd, PATH_MAX));
  const std::string bare_path = "src/mavsdk/plugins/mavlink_passthrough";
  const std::string dotslash_path = "./" + bare_path;
  const std::string canonical_path = std::string(cwd) + mavsdk::path_separator + bare_path;
  