During a common-mode test, the SG terminal of the impulse noise tester and the building-side earth are connected to the ground plane. Can impulse noise flow back into the building side?

The output from the impulse noise tester during a common-mode test forms a closed loop through:

tester ⇒ EUT ⇒ ground plane ⇒ tester (SG terminal),

so basically it does not pass through the building side.

The connection to the building side is solely for protective grounding.