test: fix parsing the mksquashfs version number

Testing with mksquasshfs 4.5.1 results in an error

    ValueError: could not convert string to float: '4.5.1'

Version 4.10 would be considered to be lower than 4.4.

Fixes: 04c9813e ("test/py: rewrite common tools for SquashFS tests")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>