unittest: allow resetting checkpoint list

[nigredo-tori]

Since we don't have a property check library in Nim yet, I sometimes find myself writing tests like this:

test "foo"
  for i in 1..N:
    checkpoint("i = $#" % $i)
    check: f(i) == g(i)

This, of course, generates a lot of output on failure. There are a few alternatives, but neither of those are satisfactory:

test "foo"
  for i in 1..N:
    let f0 = f(i)
    let g0 = g(i)
    if f0 != g0:
      checkpoint("i = $#" % $i)
      check: f0 == g0

The easiest way to mitigate this that I see is just adding something like the following to unittest:

proc resetCheckpoints* =
  checkpoints = @[]