test: fix process-each-duplicate-vgnames

After enable_dev, the following commands were not
consistently seeing the pv on it.

Alasdair explained, "whenever enabling/disabling devs
outside the tools (and you aren't trying to test how
the tools cope with suddenly appearing/disappering
devices) use "vgscan""

test/shell/process-each-duplicate-vgnames.sh

@@ -18,15 +18,22 @@ aux disable_dev $dev1
 aux disable_dev $dev2
 
 aux enable_dev $dev1
+vgscan
 vgcreate $vg1 $dev1
 UUID1=$(vgs --noheading -o vg_uuid $vg1)
 aux disable_dev $dev1
 
 aux enable_dev $dev2
+vgscan
 vgcreate $vg1 $dev2
 UUID2=$(vgs --noheading -o vg_uuid $vg1)
 
 aux enable_dev $dev1
+# need vgscan after enabling/disabling devs
+# so that the next commands properly see them
+vgscan
+pvs $dev1
+pvs $dev2
 
 vgs -o+vg_uuid >err
 cat err
@@ -43,6 +50,7 @@ cat err
 grep $UUID1 err
 not grep $UUID2 err
 aux enable_dev $dev2
+vgscan
 
 aux disable_dev $dev1
 vgs -o+vg_uuid >err
@@ -50,4 +58,5 @@ cat err
 grep $UUID2 err
 not grep $UUID1 err
 aux enable_dev $dev1
+vgscan