Relax the block count check on deallocation in env_test

It seems that on some FS we get more blocks than we ask for. This is already handled when checking the allocated number of blocks, but after the file is closed it checks for an exact number of blocks, which fails on my machine.

I changed the test to add one full page to the size, then calculate the expected number of blocks and check if the actual number of blocks is less or equal to that.
I don't really understand how does the block allocation work, so it's hard to say which version is correct. The point of mine was to just extend the size by some not-completely-arbitrary constant, so that the number of blocks will be always lower than that.