tests: select-tools.sh: pvchange now uses process_each_pv

tests: select-tools.sh: pvchange now uses process_each_pv

pvchange now uses process_each_pv so uncomment parts of the test
which check proper functionality of intersection between selection
result and PVs or PV tags directly provided on command line. This
didn't work properly before when pvchange was not using process_each_pv.

For example:

  pvchange -u -S 'pv_name=/dev/sda' /dev/sdb

..changes nothing since clearly the intersection of /dev/sda and
/dev/sdb is empty set. The same applies for tags:

  pvchange -u -S 'pv_name=/dev/sda' @some_tag

..changes nothing if /dev/sda is not tagged with some_tag.