Make FileStore tests more tolerant.

The root filesystem should almost never have zero unallocated
blocked even when built as a read-only image, but it seems
to happen occasionally, so tolerate it.

Fixes: 222049132
Test: atest libcore.java.nio.file.LinuxFileSystemTest
Change-Id: Ic916ad546f80238f15ba1001d24b7be62129905d
diff --git a/luni/src/test/java/libcore/java/nio/file/LinuxFileSystemTest.java b/luni/src/test/java/libcore/java/nio/file/LinuxFileSystemTest.java
index ce25ed3..54d9b43 100644
--- a/luni/src/test/java/libcore/java/nio/file/LinuxFileSystemTest.java
+++ b/luni/src/test/java/libcore/java/nio/file/LinuxFileSystemTest.java
@@ -104,7 +104,7 @@
         FileStore filestore = getFilestore("/");
         assertEquals(0, filestore.getBlockSize() % 512);
         assertTrue(filestore.getTotalSpace() > 0);
-        assertTrue(filestore.getUnallocatedSpace() > 0);
+        assertTrue(filestore.getUnallocatedSpace() >= 0);
         assertTrue(filestore.getUsableSpace() >= 0);
     }