8331999: BasicDirectoryModel/LoaderThreadCount.java frequently fails on Windows in CI

Introduce tolerance factor: count > loaderCount.size() / 2
Fail the test only if the number of snapshots with more than
2 file loader threads is greater than half of the number
of valid snapshots.

Reviewed-by: prr, honkar

test/jdk/javax/swing/plaf/basic/BasicDirectoryModel/LoaderThreadCount.java

@@ -164,7 +164,7 @@ public final class LoaderThreadCount extends ThreadGroup {
             System.out.println("  = 1: " + ones);
             System.out.println("  = 2: " + twos);
             System.out.println("  > 2: " + count);
-            if (count > 0) {
+            if (count > loaderCount.size() / 2) {
                 throw new RuntimeException("Detected " + count + " snapshots "
                                            + "with several loading threads");
             }