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
1 file changed