Try somehting like this
Get-VM | New-HardDisk -DiskType "flat" -StorageFormat "Thin" -CapacityKb 1024 -Datastore "MyDS" -Confirm:$false -DiskPath "[MyDS] Test/" + $_.Name + "_test.vmdk"
Note that I added an explicit .vmdk name. That will make it easier to remove the .vmdk later on.
The remove can be done with something like this
Get-VM | Get-HardDisk | Where {$_.Filename -like "[MyDS] Test/*" | Remove-HardDisk -Confirm:$false
You will have to create the folder "[MyDS] Test" before of course !