Extract C++17-only content from unit-items.cpp into its own test file

unit-items.cpp is a 1433-line file that was being compiled twice per
CI configuration (once for C++11, once for C++17) purely because it
contained a single, small JSON_HAS_CPP_17-gated SECTION ("structured
bindings", 14 lines). Move that SECTION into a new, dedicated file
(tests/src/unit-items-cpp17.cpp) so only that tiny file needs a
second build; unit-items.cpp itself now builds/tests only once. No
tests/CMakeLists.txt changes are needed since the existing
file(GLOB ... src/unit-*.cpp) plus json_test_add_test_for() already
auto-register and standard-gate any new unit-*.cpp file based on
whether it textually contains JSON_HAS_CPP_<N> (the same mechanism
already used for the existing unit-iterators3.cpp file, which follows
the identical pattern).

Verified with plain clang++ under -std=c++11/14/17/20 and via a local
CMake configure+build that:
- unit-items.cpp now only produces a test-items_cpp11 target (the
  former test-items_cpp17 target is gone) and its assertion/test-case
  counts are unchanged (2 test cases / 222 assertions) for every
  standard.
- The new unit-items-cpp17.cpp produces test-items-cpp17_cpp11 (an
  intentionally empty translation unit under C++11 that reports 0
  tests, 0 assertions, SUCCESS) and test-items-cpp17_cpp17 (1 test
  case / 1 assertion, identical to what "structured bindings" ran
  as before it was moved).

Separately, unit-regression1.cpp (1530 lines) was also being built
twice per CI configuration because it contained the substring
JSON_HAS_CPP_17 -- but on inspection this was dead code: an orphaned
"#ifdef JSON_HAS_CPP_17 / #include <variant> / #endif" left over from
when the actual std::variant-based regression test (issue #1292) was
relocated to unit-regression2.cpp. Nothing in unit-regression1.cpp
uses <variant>, so there is no SECTION/TEST_CASE to preserve here;
the dead include is simply removed. This was verified by grepping the
file for any other use of "variant" (none) and confirming issue #1292
is still covered by unit-regression2.cpp. Compiled and ran under
-std=c++11/14/17/20 and via CMake: unit-regression1.cpp now only
produces a test-regression1_cpp11 target (test-regression1_cpp17 is
gone) with an unchanged test-case count (3) under every standard.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-items-cpp17.cpp

@@ -0,0 +1,42 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-items.cpp (structured
+// bindings support for json::items()). It is kept in a separate
+// translation unit so the (much larger) unit-items.cpp does not need to
+// be compiled a second time just for this one SECTION.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#ifdef JSON_HAS_CPP_17
+    #include <map>
+    #include <string>
+
+TEST_CASE("items()")
+{
+    SECTION("object")
+    {
+        SECTION("structured bindings")
+        {
+            json j = { {"A", 1}, {"B", 2} };
+
+            std::map<std::string, int> m;
+
+            for (auto const&[key, value] : j.items())
+            {
+                m.emplace(key, value);
+            }
+
+            CHECK(j.get<decltype(m)>() == m);
+        }
+    }
+}
+#endif

tests/src/unit-items.cpp

@@ -862,22 +862,6 @@ TEST_CASE("items()")
 
             CHECK(counter == 3);
         }
-
-#ifdef JSON_HAS_CPP_17
-        SECTION("structured bindings")
-        {
-            json j = { {"A", 1}, {"B", 2} };
-
-            std::map<std::string, int> m;
-
-            for (auto const&[key, value] : j.items())
-            {
-                m.emplace(key, value);
-            }
-
-            CHECK(j.get<decltype(m)>() == m);
-        }
-#endif
     }
 
     SECTION("const object")

tests/src/unit-regression1.cpp

@@ -30,10 +30,6 @@ using nlohmann::json;
 #include <cstdio>
 #include "make_test_data_available.hpp"
 
-#ifdef JSON_HAS_CPP_17
-    #include <variant>
-#endif
-
 #include "fifo_map.hpp"
 
 /////////////////////////////////////////////////////////////////////