tests: Fixing bug in tests-common.sh

There was a copy-paste error with the drop-off dir variable. This
commit fixes that.
spd_readdir: Cache directories in chunks

This commit patches the spd_readdir preload and extends it, so it is
able to read directories in chunks. This can be set up using a
environment variable.
fs_benchmark: Support spd_readdir cached modes
tests: Adding some more information to the cp test
fixup
ext4_comparison: Adding new script

This patch adds a script that is suitable for making comparisons of
different settings of the ext4 file system.
tests: Adding perf and seekwatcher test classes
Makefile: Fixing clean target