Add custom assertions about luaunit

[icamys]

@bluebird75 The point is that if we have a fixture data set of 100 entries and we use a loop to run a test with those fixture entries and the test fails on 85th entry, it would be great to add to message at least the number of the entry.
At the moment we need to eliminate each of 100 entries to find the one that causes test failure

[bluebird75]

I am working on my own patch for this, with small changes to what you provided. I want to review this deeply because it affects a big number of functions.

[bluebird75]

I need to give it some thought. I find the default assertion messages quite good already, not sure we need to have custom messages.

But for assertions inside a loop, you do have a point.

[bluebird75]

Done with luaunit v3.3