// ensures that two consecutive runs are unlikely to clash.
class ModularFileSystemTest : public ::testing::TestWithParam<std::string> {
 public:
  // Initializes `root_dir_` to a unique value made of `::testing::TempDir()`, a
  // static random value unique for all the tests in one invocation, and the
  // current test name.
  //
  // Since the test name contains `/` (due to parameters), this function