Revert PR #684

Somehow it's breaking test 59 of S06-advanced/wrap.t

    1..2
    ok 1 - '{nextsame}()' died
    not ok 2 - right exception type (X::NoDispatcher)
    # Expected: X::NoDispatcher
    # Got:      X::ControlFlow::Return
    # Exception message: Attempt to return outside of any Routine

Since I have *no* idea on how the patch can have this effect, and
we cannot change the spectest just for that, I felt I had to revert.

lib/Test.pm6

@@ -289,12 +289,7 @@ sub skip-rest($reason = '<unknown>') is export {
     $time_before = nqp::time_n;
 }
 
-multi sub subtest($desc, &subtests) is export {
-    subtest(&subtests, $desc);
-}
-
-multi sub subtest(&subtests, $desc = '') is export {
-    diag($desc) if $desc;
+sub subtest(&subtests, $desc = '') is export {
     _push_vars();
     _init_vars();
     $indents ~= "    ";