A compression test can certainly be done without opening
the intake air, but in a test you want the HIGHEST possible reading to test the components [ rings,valves, etc.]
So restricting the intake air passage on any gas engine
may give an inaccurate reading.
At cranking speed, there May be enough intake air to get a good reading, but open intake [ throttle/choke/etc.] assures there is.

Common tech pratice .

http://www.w-p-c.com/buggycompression.htm