Experiment1.sol

pragma solidity ^0.4.8;

contract Experiment1 {
    event LogDebug(uint val);

    function trigger(uint[] a, bytes32 b) external {
        LogDebug(a.length);
    }
}

TestExperiment1.js

'use strict';

let Experiment1 = artifacts.require('./Experiment1.sol');

contract('Experiment1', function(accounts) {

  it('should log input array of length 2', function () {
    let instance;

    return Experiment1.deployed().then(function (inst) {
      instance = inst;
      let a = [web3.sha3('one'), web3.sha3('two')];
      let b = Buffer.alloc(32);
      return instance.trigger(a, b, { from: accounts[0] });

    }).then(function (res) {
      console.log('Logs:', res.logs);
      console.log('Logs[0].args:', res.logs[0].args);
      console.log('Logs[0].args.val as string:', hextostr(res.logs[0].args.val.toString(16)));
      assert.equal(res.logs[0].args.val.toString(16), '2', 'should have logged array length 2');

    });
  });
});

function hextobin(h) { return Buffer.from(h.replace('0x', ''), 'hex'); }
function hextostr(h) { return hextobin(h).toString('utf8'); }

truffle-test.log

$ truffle test
Using network 'development'.



  Contract: Experiment1
Logs: [ { logIndex: 0,
    transactionIndex: 0,
    transactionHash: '0xc5f145993081ed979c54396b319a838c028cb0b02c0b0a3cbca4b76d9cba23bd',
    blockHash: '0x9ccaf9737df1a75108474f2334954eb5febf9d6905a7d4944e261325350779e4',
    blockNumber: 647,
    address: '0xd254267d3472e90b7e18bc10aa30bad4664619ab',
    type: 'mined',
    event: 'LogDebug',
    args: { val: [Object] } } ]
Logs[0].args: { val: 
   { [String: '1.9986878972343340240046310863176586764876631823265828342326149237773911993392e+76']
     s: 1,
     e: 76,
     c: 
      [ 1998687,
        89723433402400,
        46310863176586,
        76487663182326,
        58283423261492,
        37773911993392 ] } }
Logs[0].args.val as string: ,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
    1) should log input array of length 2

    Events emitted during test:
    ---------------------------


    ---------------------------


  0 passing (77ms)
  1 failing

  1) Contract: Experiment1 should log input array of length 2:
     AssertionError: should have logged array length 2: expected '2c302c302c302c302c302c302c302c302c302c302c302c302c302c302c302c30' to equal '2'
      at test/TestExperiment1.js:20:14
      at process._tickDomainCallback (internal/process/next_tick.js:129:7)

truffle.js

let Experiment1 = artifacts.require("./Experiment1.sol");

module.exports = function(deployer) {
  deployer.deploy(Experiment1);
};