Great question, David. I would say if testing could be done in Xen, then it could likely be done in Qubes. It's really difficult to mess with dom0 or how it looks, so I doubt you will have luck switching views. What guest OS will you mainly use for testing? One option may be, if you're accustomed to Virtualbox for Windows for example, setting up a Windows VM how you like it for testing and loading a guests in there. I can't comment on the performance of Virtualbox inside of a VM though. Has anyone else done this?
The tool made for this is vagrant. https://www.vagrantup.com/
most vagrant boxes are command line only. for gui desktops, theres boxcutter/ubuntu1604-desktop and mwrock/Windows2012R2
not that you cant make hvm templates in qubes and go with that, but you wont be able to share or port your development environment as easily.
qubes is awsome in other ways, and once you try it, you wont want to go back. but, nested virtualization is disabled in qubes for security reasons. so you wont get to use vagrant in its default form. theres an lxc plugin for vagrant, linux only, and you could use the libvirt plugin with qemu, which would be in emulation, which is really slow. virtualbox 32bit might also work, but would also be slow (emulation) if it did.
if it matters, another limitation is the lack of graphics acceleration, but its still fast enough for most 2d tasks and watching movies in full screen on most laptops.
if you have a reliable connection to something you can use as a vagrant server, id use qubes as a terminal to that (which we do at work). if not, and if you want to be able to easily share your development environment, id use linux or whatever desktop your comfortable with, vagrant, and virtualbox or kvm if you need nesting in your environments (if your testing hypervisors for example)
if the dev environment is trivial or sharing it doesnt matter so much, then you might as well benefit from qubes.
> P.S. If this is possible, Qubes OS also seems like a more flexible
> alternative to dual-booting?
dual booting would break the security model. if you do want to dual boot, look into AEM to make sure the other os doesnt compromise the boot loader for qubes.