tests/unit-iterators2: use std::ranges::equals for range comparisons

[ArsenArsen]

This fixes a build error on GCC 13:

/home/arsen/nlohmann_json/tests/src/unit-iterators2.cpp: In function ‘void _DOCTEST_ANON_FUNC_2()’: /home/arsen/nlohmann_json/tests/src/unit-iterators2.cpp:946:32: error: no match for ‘operator==’ (operand types are ‘doctest::detail::Expression_lhs<std::ranges::reverse_view<std::ranges::ref_view<nlohmann::json_abi_v3_11_2::basic_json<> > >&>’ and ‘nlohmann::json_abi_v3_11_2::json’ {aka ‘nlohmann::json_abi_v3_11_2::basic_json<>’}) 946	CHECK(reversed == j_expected);	^~ ~~
nlohmann::json_abi_v3_11_2::json {aka nlohmann::json_abi_v3_11_2::basic_json<>}

Bug: https://bugs.gentoo.org/895692 Closes: #3927

This PR also depends on trofis PR: #3895

Tested on x86_64-pc-linux-gnu in the default CMake configuration.

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• [x] Changes are described in the pull request, or an existing issue is referenced.
• [x] The test suite compiles and runs without error.
• [x] Code coverage is 100%. Test cases can be added by editing the test suite.
• [x] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
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[coveralls]

Coverage: 100.0%. Remained the same when pulling a49829bd984c0282be18fcec070df0c31bf77dd5 on ArsenArsen:gcc13 into b2306145e1789368e6f261680e8dc007e91cc986 on nlohmann:develop.

[nlohmann]

Thanks!