Fix compatibility with CMake < 3.14

[Flamefire]

NAME_WLE was introduced in 3.14

I'm quite sure that "longest" and "last" extension for all tests are the same but please double check.

BTW: The 2 target_include_directories(pfr_core_${testname} PRIVATE ../../../) look suspicious. Why are they required when you already link to Boost::core and Boost::pfr?

[coveralls]

Pull Request Test Coverage Report for Build 18129831217

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 17710819055:	0.0%
Covered Lines:	405
Relevant Lines:	405

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💛 - Coveralls

[apolukhin]

Many thanks for the fix!