hello_eth_salon.js

var HelloEthSalon = artifacts.require("./HelloEthSalon.sol");

contract("HelloEthSalon:GetMessage", function (accounts) {
  it("should return a correct string", async function () {
    const contract = await HelloEthSalon.deployed();
    const result = await contract.GetMessage.call();
    assert.isTrue(result === "I know smart contract testing!!");
  });
});

here is callback version (in my old tutorials)

var HelloEthSalon = artifacts.require('./HelloEthSalon.sol');

contract('HelloEthSalon:GetMessage', function(accounts) {
  it("should return a correct string", function(done) {
    var hello_eth_salon = HelloEthSalon.deployed();
    hello_eth_salon.then(function(contract){
      return contract.GetMessage.call();
    }).then(function(result){
      assert.isTrue(result === 'I know testing of a contract!!');
      done();
    })
  });
});