exclude some parts of the tests from the build

[EfesX]

I've excluded the test parts that only work if JSON_DisableEnumSerialization option is OFF.

4384

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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.

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[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @EfesX Please read and follow the Contribution Guidelines.

[coveralls]

coverage: 100.0%. remained the same when pulling 6d75a9fb0b1dd79abace90859a7a3a8fe86ddf5e on EfesX:issue_4384_fix_build_noenum_test into 8c391e04fe4195d8be862c97f38cfe10e2a3472e on nlohmann:develop.

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @EfesX Please read and follow the Contribution Guidelines.

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @EfesX Please read and follow the Contribution Guidelines.

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @EfesX Please read and follow the Contribution Guidelines.

[EfesX]

Hmm, this seems to be the expected behavior... I should have read the docs more carefully )

JSON_DISABLE_ENUM_SERIALIZATION

When defined to `1`, default serialization and deserialization functions for enums are excluded and have to be provided by the user, for example, using [`NLOHMANN_JSON_SERIALIZE_ENUM`](nlohmann_json_serialize_enum.md) (see [arbitrary type conversions](../../features/arbitrary_types.md) for more details).