filedatabase.cpp: Fix return type and don't leak

• Add Tests
• Port cxx_filedatabase.cpp tests
• link against -lstdc++fs for GCC < 9, see https://stackoverflow.com/questions/48729328/link-errors-using-filesystem-members-in-c17
• Port filedatabase.cpp to catch as !1266 (merged) adds catch2 support for filedatabase tests
• Add tests for FileDatabase::DBPutProperty
• Add FileDatabase::DBPutProperty abd FileDatabase::DBDeleteProperty

target_link_libraries(mylibrary PRIVATE $<$<AND:$<CXX_COMPILER_ID:GNU>,$<VERSION_LESS:$<CXX_COMPILER_VERSION>,9.0>>:stdc++fs>)

https://gitlab.kitware.com/cmake/cmake/-/issues/17834

@phako please give that a shot.

Close #1261