move passing stage1 compile error tests to stage2

test/cases/compile_errors/unreachable_code.zig

@@ -0,0 +1,13 @@
+export fn a() void {
+    return;
+    b();
+}
+
+fn b() void {}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:6: error: unreachable code
+// :2:5: note: control flow is diverted here