Add the option to run tests with ext2 and ext3 partitions

Spoiler alert: they don't pass...
0015: new testcase to test single indirect blocks

There's a bug on single indirected blocks.  This is only relevant for
non-ext4 partitions (or ext4 partitions that were converted), since pure
ext4 only will use extents.
Fix ext2/ext3 single indirection file reading

I never tested non-ext4 partitions and this is the proof.  Since ext4
doesn't use indirected blocks, the code for it was simply broken.  Now,
files under 80KB should work fine (t0015 now passes with ext2/3), but
larger files are still unsupported.

This support is also relevant for partitions that have been upgraded from
older formats to ext4 (via tune2fs).  Newly formatted ext4 should never hit
this code path since they always rely on extents.