test(tvix/eval): builtins.sort must preserve order of equal elements

Change-Id: I59a0756940d1e5360a2ab4e886cf0bc9af7b8901
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7133
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.exp

@@ -0,0 +1 @@
+[ { index = 7; key = 0; } { index = 0; key = 1; } { index = 13; key = 1; } { index = 1; key = 2; } { index = 3; key = 2; } { index = 4; key = 2; } { index = 5; key = 2; } { index = 12; key = 2; } { index = 14; key = 2; } { index = 2; key = 3; } { index = 11; key = 3; } { index = 15; key = 3; } { index = 10; key = 4; } { index = 6; key = 5; } { index = 8; key = 5; } { index = 9; key = 5; } { index = 16; key = 22; } ]

tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.nix

@@ -0,0 +1,7 @@
+let
+  keys = [ 1 2 3 2 2 2 5 0 5 5 4 3 2 1 2 3 22 ];
+in
+
+builtins.sort
+  (a: b: a.key < b.key)
+  (builtins.genList (index: { inherit index; key = builtins.elemAt keys index; }) (builtins.length keys))