Testing for freestanding

[dimdin]

You don’t need the _entry in test_main.zig.
Keep only one _start (in main.zig, or somewhere that both main.zig and test_main.zig load) as:

// root is: root_source_file (main.zig) for normal run,
//       or test_runner (test_main.zig) for test run.
const root = @import("root");

export fn _start() noreturn {
    // do essential init work
    arch.platform.setup();
    if (builtin.is_test) {
        root.tmain();
    } else {
        root.kmain();
    }
    unreachable;
}