test: convert most N-API tests from C++ to C

• Prefix functions with static to make them local
• Remove anonymous namespaces
• nullptr -> NULL
• .cc -> .c and update binding.gyp
• static_cast<x>() -> (x)()
• Replace new/delete with malloc()/free() (only in test_callback_scope)
• Move lambda out and convert to local function (only in test_callback_scope)
• Remove superfluous #include <vector> (only in test_callback_scope_recurse)

Some tests are best left as C++.

ls -l test/{node-api,js-native-api}/*/*.cc

for the rest.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines