Add test for DRBD 8.4 /proc/drbd parsing

The current parser already supports the output of DRBD 8.4 /proc/drbd,
so add tests which actually verify this behaviour.
Note that the empty lines in proc_drbd84.txt are actually present in the
output of DRBD 8.4, they always appear when minors are not contiguous.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>