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.

Backport-of: ffbdfffbc702253f32fa45dc1855b663c72074a6

Author: Andrew Lu

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

@@ -164,7 +164,7 @@ private static void runTest(final long timeStart) throws Throwable {
             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");
             }