[perl #128123] [JVM] failing test in S03-metaops/hyper.t after change from postfix:<++> to prefix:<++>

[Christian Bartolomaeus via RT]

And now a lot of tests from roast exploded with "Expected a native int argument 
for '$a'" after this commit introduced two uses of postfix:<++> and 
postfix:<--> in lib/Test.pm6: 
https://github.com/rakudo/rakudo/commit/ffb5789f7eef1157c7556897c4805569df4f7aa4

Modifying the two relevant lines seems to fix those failures (did not run full 
spectest yet):

diff --git a/lib/Test.pm6 b/lib/Test.pm6
index 45bb86f..5d052e9 100644
--- a/lib/Test.pm6
+++ b/lib/Test.pm6
@@ -339,9 +339,9 @@ multi sub subtest(&subtests, $desc = '') is export {
     _push_vars();
     _init_vars();
     $indents ~= "    ";
-    $subtest_level++;
+    $subtest_level += 1;
     subtests();
-    $subtest_level--;
+    $subtest_level -= 1;
     done-testing() if nqp::iseq_i($done_testing_has_been_run,0);
     my $status =
       $num_of_tests_failed == 0 && $num_of_tests_planned == $num_of_tests_run;