If you just want to test, why not download the Enterprise evaluation version of 10.10240:
Windows 10 Enterprise | TechNet Evaluation Center.
It is personalizable, functionally equivalent to Professional, and has a 90 days evaluation period which may be extended twice with rearms. Within the limits of the license, it is activated, but it does have a watermark. There are minor differences in Windows setup regarding whether you join an enterprise domain, but you just need to choose the alternative.
You can remain in the insider program, and receive fast or slow track updates as far as the advanced updates settings page shows, as long as you log in with your MSA, and if you have other systems (even 8.1) logged in with the same MSA, your settings follow you around.
Frankly I am not concerned about Activation status on insider builds - it's not as if I feel guilty at running some illicit system, but I guess we have become somewhat sensitized to it over the years - just like the lack of the Start menu.
The only system I installed since 17th July that activated was performed after the activation servers were supposedly downed was on an installation of 10.10166 that I upgraded immediately, without activating first, to 10.10240. Like previous upgrades it was a partial success, and I had to reset it, but it remained activated.