Improve serialization method, add comprehensive tests

[#43089109]
Move MML and expected JSON files to test/fixtures

[#43089109]

In the future this set of files can be useful for Mocha tests also.
Fix and reenable deserialize-serialize tests

[#43089109]

There is one thing missing - comparision of custom LJ properties
array. There are two tricky things:
- order of objects in this array is unimportant,
- objects contain 'element1' and 'element2' properties and it's
not important whether 'element1' = X and 'element2' = Y or inversely.
So, assert.deepEqual is not enought to cover this case.