remove deprecated stuff in unittest module

[ringabout]

• use runnableExamples
• remove one callsite
• add two regression tests

[timotheecour]

after removing callsite, CI fails in in case: https://github.com/LemonBoy/jstin with nim r tests/test1.nim

after reduction, here's a repro:

import unittest
type MyFoo = object
var obj = MyFoo()
let check = 1
check(obj == obj)

/Users/timothee/git_clone/nim/temp/jstin/tests/test1.nim(16, 6) template/generic instantiation from here
/Users/timothee/git_clone/nim/temp/jstin/tests/test1.nim(19, 6) template/generic instantiation from here
/Users/timothee/git_clone/nim/Nim_prs/lib/pure/unittest.nim(706, 9) template/generic instantiation from here
/Users/timothee/git_clone/nim/temp/jstin/tests/test1.nim(16, 6) template/generic instantiation from here
/Users/timothee/git_clone/nim/temp/jstin/tests/test1.nim(19, 6) template/generic instantiation from here
/Users/timothee/git_clone/nim/Nim_prs/lib/pure/unittest.nim(706, 9) template/generic instantiation from here
/Users/timothee/git_clone/nim/Nim_prs/lib/pure/unittest.nim(691, 46) Error: internal error: expr(nkIdent); unknown node kind
              result.printOuts.add getAst(print(argStr, arg))
                                               ^
/Users/timothee/git_clone/nim/Nim_devel/compiler/ccgexprs.nim(2913, 22) compiler msg initiated here [MsgOrigin]

and here's a different issue with a similar test case:

when true:
  import unittest
  block:
    let check = 123
    var a = 1
    var b = 1
    check(a == b)

/Users/timothee/git_clone/nim/Nim_devel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /Users/timothee/git_clone/nim/Nim_devel/compiler/ccgexprs.nim(616, 9) `e[1].typ != nil`  [AssertionDefect]

maybe those are fixable.

Another aspect is whether we actually should deprecate callsite or not; I've argued in #11864 (comment) that maybe we shouldn't.

regardless, if we can avoid callsite here, it might be cleaner.

note

can you add those 2 minimized examples to tunittest to avoid future regressions?