Reduce memory use in threading correctness tests (#3172)

Author: yakir12

test/indexing.jl

@@ -2023,15 +2023,13 @@ include("indexing_offset.jl")
 
 @testset "threading correctness tests" begin
     for x in (10, 1_100_000), y in 1:4
-        mat = rand(Int8, x, y)
-        df = DataFrame(mat, :auto)
+        vecvec = [rand(Int8, x) for _ in 1:y]
+        df = DataFrame(vecvec, :auto, copycols=false)
         for rowrange in [:, 1:nrow(df)-5, collect(1:nrow(df)-5), axes(df, 1) .< nrow(df)-5],
             colrange in [:, axes(df, 2), collect(axes(df, 2)), 1:ncol(df) - 1]
-            # Equivalent but using less memory than:
-            # @test DataFrame(mat[rowrange, colrange], :auto) == df[rowrange, colrange]
             df2 = df[rowrange, colrange]
             for j in axes(df2, 2)
-                @test df2[!, j] == view(mat, rowrange, j)
+                @test df2[!, j] == view(vecvec[j], rowrange)
             end
         end
     end