test/py: Improve check for mksquashfs version

Some builds of squashfs-tools append version string with "-git" or
similar. The float() conversion will fail in this case.

Improve the code to only convert to float() the string before the '-'
character.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joao Marcos Costa <jmcosta944@gmail.com>