tests: pvscan-cache try to fix teardown problems

teardown after the test was failing, probably because
of uncoordinated udev actions running on the test
system.  Try to avoid this by doing some work before
teardown.

test/shell/pvscan-cache.sh

@@ -75,4 +75,7 @@ aux enable_dev "$dev2"
 pvscan --cache
 check pv_field "$dev1" vg_name "$vg1"
 
+vgchange -an $vg1
+lvremove $vg1/$lv1
+
 vgremove -ff $vg1