Show on which line assertion failed

[kristijanhusak]

Consider having a test like this:

let s:suite = themis#suite('my_test')
let s:assert = themis#helper('assert')

function! s:suite.test_valid() abort
  call s:assert.equals(0, 1-1)
  call s:assert.equals(2, 1+1)
  call s:assert.equals(3, 1+1)
  call s:assert.equals(4, 2+2)
  call s:assert.equals(5, 3+2)
endfunction

Output for spec reporter is this:

my_test
  [✖] test_valid
      The equivalent values were expected, but it was not the case.

          expected: 2
               got: 3

tests 1
passes 0
fails 1

There's no information which exact assertion (on which line) failed.
Same thing with other reporters.

Something like this would be great:

my_test
  [✖] test_valid
      The equivalent values were expected, but it was not the case.

          expected: 2
               got: 3
		      line: 7

tests 1
passes 0
fails 1