test: include lua headers in extern "C" scope

Lua headers don't have C++ guards which means all Lua symbols are
mangled as C++ ones when the files included in a C++ source file,
but then the linker fails because all Lua functions are exported
with C mangling. Let's include lua headers in extern "C" scope to
allow to use "lua_test_utils.h" in unit tests written in C++.

NO_CHANGELOG=test
NO_DOC=test