Fix one test to pass GL pipeline

[mattdowle]

follow up to pr #3124
fixed test 1246.55 that failed thanks to one random seed on gitlab pipelines
Added direct test of this case to consistently cover it.

Running test id 1246.54
Running test id 1246.55    
Test 1246.55 ran without errors but failed check that x equals y:
> x = duplicated(DT, by = jj, fromLast = TRUE)
First 6 of 100 (type 'logical'): [1] FALSE FALSE FALSE FALSE FALSE FALSE
> y = duplicated.data.frame(DT[, jj, with = FALSE], fromLast = TRUE)
First 6 of 100 (type 'logical'): [1] FALSE FALSE FALSE FALSE FALSE FALSE
1 element mismatch
Running test id 1246.56
Running test id 1246.57
Running test id 1246.58
Running test id 1246.59
Running test id 1246.6
forder decreasing argument test: seed = 1540402216  colorder = 4,2,3,1colorder = 2,4,3,1
Running test id 1247.1

also simplified dtwiddle()

[codecov]

Codecov Report

Merging #3126 into master will decrease coverage by <.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #3126      +/-   ##
==========================================
- Coverage   91.96%   91.95%   -0.01%     
==========================================
  Files          61       61              
  Lines       11434    11437       +3     
==========================================
+ Hits        10515    10517       +2     
- Misses        919      920       +1

Impacted Files	Coverage Δ
src/forder.c	98.93% <93.75%> (-0.18%)	⬇️

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