devel/nlohmann-json: Update 2.1.1 -> 3.1.2

Port changes:

• Change to DISTVERSION
• Change license to MIT (it changed?)
• Sort USES
• Preserve the cmake file location set in CMakeLists.txt
• Remove the TEST option, because it can be tested without such option.
• Add the do-test target that builds tests and tests in one step

PR: 227145
Approved by: SlavaMikerov@gmail.com (maintainer's timeout; 48 days)