add a few more unit tests for functions in utils

#include "gmock/gmock.h"

#include "utils.h"

#include <string>

#include <cerrno>

#include <cstdio>

#include <cstdlib>

using namespace LAMMPS_NS;

using ::testing::Eq;

    ASSERT_THAT(utils::path_basename("/parent/folder/filename"), Eq("filename"));

#endif

}

TEST(Utils, getsyserror) {

#if defined(__linux__)

    errno = ENOENT;

    std::string errmesg = utils::getsyserror();

    ASSERT_THAT(errmesg, Eq("No such file or directory"));

#else

    GTEST_SKIP();

#endif

}

TEST(Utils, potential_file) {

    FILE *fp;

    fp = fopen("ctest.txt","w");

    ASSERT_NE(fp,nullptr);

    fputs("# DATE: 2020-02-20 CONTRIBUTOR: Nessuno\n",fp);

    fclose(fp);

    EXPECT_TRUE(utils::file_is_readable("ctest.txt"));

    EXPECT_FALSE(utils::file_is_readable("no_such_file.txt"));

    EXPECT_THAT(utils::get_potential_file_path("ctest.txt"),Eq("ctest.txt"));

    const char *folder = getenv("LAMMPS_POTENTIALS");

    if (folder != nullptr) {

      std::string path=utils::path_join(folder,"Cu_u3.eam");

      EXPECT_THAT(utils::get_potential_file_path("Cu_u3.eam"),Eq(path));

    }

    EXPECT_THAT(utils::get_potential_date("ctest.txt","Test"),Eq("2020-02-20"));

    remove("ctest.txt");

}