Hyper-V guest failover cluster file server

In order to create a true “shared-nothing” failover file server you can use a free Starwind solution https://www.starwindsoftware.com/starwind-virtual-san-free.

Basically it can do the same that Windows Server 2016 will do later but unfortunately only in the Datacenter edition. Starwind can do it already even with a free Hyper-V 2012 server https://slog.starwindsoftware.com/smb3-0-fileserver-on-free-microsoft-hyper-v-server-2012r2-clustered/.

Apparently you can take the shared VHDX offline (the shares will be unavailable), either by shutting down the clustered guests or by unmounting it, and then change the size of the VHDX and then bring it back online (or boot the guests).

A few web searches have turned up people writing and agreeing that that's the only way to do it. There's one such conversation and a powershell script here that purports to automate the process with minimum downtime.

Because this is only a testing environment have you considered creating several shared VHDX disks and using them to create a Clustered Storage Space? that way you can add more disks to the storage space to increase capacity as needed.