Fix duplicate test count id

Uses `cas(...)` instead of atomic operators/types so that this
should not break on the JVM as well as be backwards compatible
with older rakudos (these platforms will still experience
the duplicate test count id, however). This does not fix the
output order, but it does fix the plan output (thereby fixing
issues where a plan 200 is used, but the output would be 1..197

See: https://irclog.perlgeek.de/perl6-dev/2017-08-29#i_15086510

lib/Test.pm6

@@ -675,16 +675,16 @@ sub eval_exception($code) {
 sub proclaim(Bool(Mu) $cond, $desc is copy, $unescaped-prefix = '') {
     _init_io() unless $output;
     # exclude the time spent in proclaim from the test time
-    $num_of_tests_run = $num_of_tests_run + 1;
+    my $current_tests_run = cas $num_of_tests_run, -> $i { $i + 1 };
 
     my $tap = $indents;
     unless $cond {
         $tap ~= "not ";
 
-        $num_of_tests_failed = $num_of_tests_failed + 1
-            unless $num_of_tests_run <= $todo_upto_test_num;
+        cas( $num_of_tests_failed, -> $i { $i + 1 } )
+            unless $current_tests_run <= $todo_upto_test_num;
 
-        $pseudo_fails = $pseudo_fails + 1 if $subtest_todo_reason;
+        cas( $pseudo_fails, -> $i { $i + 1 } ) if $subtest_todo_reason;
     }
 
     # TAP parsers do not like '#' in the description, they'd miss the '# TODO'
@@ -696,11 +696,11 @@ sub proclaim(Bool(Mu) $cond, $desc is copy, $unescaped-prefix = '') {
                     nqp::split('#', $desc.Str))))
     !! '';
 
-    $tap ~= $todo_reason && $num_of_tests_run <= $todo_upto_test_num
-        ?? "ok $num_of_tests_run - $unescaped-prefix$desc$todo_reason"
+    $tap ~= $todo_reason && $current_tests_run <= $todo_upto_test_num
+        ?? "ok $current_tests_run - $unescaped-prefix$desc$todo_reason"
         !! (! $cond && $subtest_todo_reason)
-            ?? "ok $num_of_tests_run - $unescaped-prefix$desc$subtest_todo_reason"
-            !! "ok $num_of_tests_run - $unescaped-prefix$desc";
+            ?? "ok $current_tests_run - $unescaped-prefix$desc$subtest_todo_reason"
+            !! "ok $current_tests_run - $unescaped-prefix$desc";
 
     $tap ~= ("\n$indents# t=" ~ ceiling(($time_after - $time_before)*1_000_000))
         if nqp::iseq_i($perl6_test_times,1);