nixos/tests/installer: lvm: test lvm2-pvscan@ units
nixos/tests/installer: lvm: test lvm2-pvscan@ units

Also, add some sleep statements in between, which seems to at least feel
like it causes

> WARNING: Device /dev/vda* not initialized in udev database even after waiting 10000000 microseconds.

To occur less frequently.

This eventually still succeeds after some amount of waiting, I suspect
some racyness in the way lvm's udev-triggered scripts trigger other
units.

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