tests/functional/repl: Characterize side effecting print behavior

Reported on matrix by aleksana:
https://matrix.to/#/!VRULIdgoKmKPzJZzjj:nixos.org/$7wZp5lUDTd-_u6MYo8kWWcysjtqTiQqP8dLI0RDNVVM?via=nixos.org&via=matrix.org&via=nixos.dev

tests/functional/repl/pretty-print-idempotent.expected

@@ -0,0 +1,33 @@
+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+{
+  homepage = "https://example.com";
+}
+
+{ homepage = "https://example.com"; }
+
+{
+  layerOne = { ... };
+}
+
+{
+  layerOne = { ... };
+}
+
+[
+  "https://example.com"
+]
+
+[ "https://example.com" ]
+
+[
+  [ ... ]
+]
+
+[
+  [ ... ]
+]
+
+

tests/functional/repl/pretty-print-idempotent.in

@@ -0,0 +1,9 @@
+:l pretty-print-idempotent.nix
+oneDeep
+oneDeep
+twoDeep
+twoDeep
+oneDeepList
+oneDeepList
+twoDeepList
+twoDeepList

tests/functional/repl/pretty-print-idempotent.nix

@@ -0,0 +1,19 @@
+{
+  oneDeep = {
+    homepage = "https://" + "example.com";
+  };
+  twoDeep = {
+    layerOne = {
+      homepage = "https://" + "example.com";
+    };
+  };
+
+  oneDeepList = [
+    ("https://" + "example.com")
+  ];
+  twoDeepList = [
+    [
+      ("https://" + "example.com")
+    ]
+  ];
+}