Fix creating TESTDISK_DIR/SHELLPACK_TMP in default case

The directory is not created if
- TESTDISK_PARTITION is not set
- TESTDISK_DIR is not set in config and
- uses SHELLPACK_TEMP as default value and

Signed-off-by: David Sterba <dsterba@suse.cz>
Add support for fio

FIO is a flexible and configurable tool to test IO. This is only a basic
support. Use in config as:

export FIO_CMD_LINE='...'
- commandline passed directly to fio

You can transform a job file into commandline arguments via
$ fio --showcmd jobfile.fio

From mmtests invoked like this:
$ fio --directory $TESTDISK_DIR $FIO_CMD_LINE

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Mel Gorman <mgorman@suse.de>