Clear interruption to avoid UpfrontAllocatingPageSourceTest failure

Test testInterruption in UpfrontAllocatingPageSourceTest intermittently
fails with an InterruptionException at the latch.await() statement.
This failure exists because a test in FileBackedStorageEngineTest
interrupts the test thread and does not clear it.  This commit clears
the interruption state.

src/test/java/org/terracotta/offheapstore/disk/storage/FileBackedStorageEngineTest.java

@@ -220,6 +220,7 @@ public FileChannel getReadableChannel() {
       Assert.assertArrayEquals(buffer, bytes);
     } finally {
       source.close();
+      Thread.interrupted();   // Clear interruption stats
     }
   }