tests: (functions.sh) add a helper funcion making a device number from given major...

tests: (functions.sh) add a helper funcion making a device number from given major and minor nums

Fixes #2919.
Suggested by Karel Zak <kzak@redhat.com>.

The original code used an obsolete formula to make a device number from
given major and minor numbers.

ts_device_make is a new helper function based on the formula of
__SYSMACROS_DEFINE_MAKEDEV macro defined in
/usr/include/bits/sysmacros.h of GNU libc.

Suggested by Karel Zak <kzak@redhat.com> in #2919.
Signed-off-by: Masatake YAMATO <yamato@redhat.com>