test_runner: keep unnamed refAllDecls tests out of the slowest list

The Zig 0.16 port of SlowTracker.endTiming dropped the early return for
unnamed tests and discarded the flag, so the refAllDecls blocks were
timed and queued like real tests. With a filter that matches nothing they
were the only candidates and filled the "Slowest 5 tests" output.

src/test_runner.zig

@@ -345,7 +345,9 @@ const SlowTracker = struct {
         const start = self.start;
         self.start = timestamp;
         const ns: u64 = @intCast(start.durationTo(timestamp).toNanoseconds());
-        _ = is_unnamed_test;
+        if (is_unnamed_test) {
+            return ns;
+        }
 
         var slowest = &self.slowest;