Fix potential "test run out of order" in 01-thread.t

Without synchronization, it may happen that two ok() run concurrently and
confuse the test count with one ok() coming in between the update of the global
test counter and the actual print of the test result.

t/concurrency/01-thread.t

@@ -25,11 +25,14 @@ plan(24);
 
 # 4 tests
 {
+    my int $done := 0;
     my $t := nqp::newthread({
-        ok(1, 'Can write to STDOUT in child thread')
+        1 until $done;
+        ok(1, 'Can write to STDOUT in child thread');
     }, 0);
     ok(1, 'Can write to STDOUT in parent thread before threadrun');
     nqp::threadrun($t);
+    $done := 1;
     ok(1, 'Can write to STDOUT in parent thread after threadrun');
     nqp::threadjoin($t);
     ok(1, 'Can write to STDOUT in parent thread after threadjoin');