This commit adds a new test case which carries out the following steps:
* write() to a block of an ext2 file, verify the write() was successful
* read() the same block back, verify the read() was successful
* verify that the data from the read() is identical to the data that was
written in the write()
The test runs the above steps on the following blocks of an ext2 file:
* the first and last direct blocks
* the first and last singly indirect blocks
* the first and last doubly indirect blocks
* the first and last triply indirect blocks
Previously, attempting to update an ext2 inode with a UID or GID
larger than 65535 would overflow. We now write the high bits of UIDs
and GIDs to the same place that Linux does within the `osd2` struct.
