To my understanding, Microsoft officially endorses the use of the unactivated "grace period" in a pure testing environment. So, if your testing is short term, you may not need to activate at all.
Microsoft says:
Leverage the Activation Grace Period
If activation does not occur
immediately after the operating system
is installed, Windows 7 and Windows
Server 2008 R2 still provide the full
functionality of the operating system
for an initial grace period of 30
days. During this grace period, at
each log in and at common intervals, a
notification appears reminding
customers to activate the product,
but otherwise, the operating system
functions the same as an activated
product.
If you have test systems that change
frequently, consider taking advantage
of the activation grace period and not
activating the product while you’re
testing it. If your testing goes
beyond 30 days but is still
short-term, Microsoft provides a way
to reset the grace period up to three
times using rearm functionality
available through the slmgr.vbs
command-line interface. This
effectively extends the grace period
of these products to 120 days. For
details see Slmgr.vbs Options in the
Volume Activation Technical Reference
Guide.
This should be legally in the clear as long as you have any kind of Windows license that you are not actively using on another device - an OEM license for the same edition would be fine. This is technically in the clear no matter what, you are simply not required to activate for 120 days.
