test(tvix/eval): check inner forcing despite declaring pointer

Maybe counter-intuitively the inner elements of a list or the
attribute values of an attribute set will be forced despite
pointer equality (but only one layer deep).

Change-Id: I485d96452fb56f5fb342d39039c9137725b33d3f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7371
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-fail-force-before-value-pointer-equality.nix

@@ -0,0 +1,5 @@
+let
+  x = throw "I have been forced";
+in
+
+x == x

tvix/eval/src/tests/tvix_tests/eval-fail-outer-value-never-pointer-equal.nix

@@ -0,0 +1,7 @@
+# For an explanation of this behavior see //tvix/docs/value-pointer-equality.md
+let
+  x = { foo = throw "foo"; };
+in
+
+# while `builtins.seq x null` would succeed, this fails!
+x == x