test runner: prepare stage2 workaround

This provides a simpler test runner that exercises fewer language
features so that we can get to the point faster where `zig test` works
for stage2.

lib/std/special/test_runner.zig

@@ -22,6 +22,9 @@ fn processArgs() void {
 }
 
 pub fn main() anyerror!void {
+    if (builtin.zig_is_stage2) {
+        return main2();
+    }
     processArgs();
     const test_fn_list = builtin.test_functions;
     var ok_count: usize = 0;
@@ -123,3 +126,10 @@ pub fn log(
         std.debug.print("[{s}] ({s}): " ++ format ++ "\n", .{ @tagName(scope), @tagName(message_level) } ++ args);
     }
 }
+
+pub fn main2() anyerror!void {
+    // Simpler main(), exercising fewer language features, so that stage2 can handle it.
+    for (builtin.test_functions) |test_fn| {
+        try test_fn.func();
+    }
+}