Inspired by this post: https://www.reddit.com/r/spaceporn/comments/6ltw1h/minuteman_iii_icbm_reenters_atmosphere_in/
My understanding is that, during the Cold War, the two countries were monitoring each other for launches at all times, ready to so some mutually assured destructing. How do you test the platform under that circumstance?
There were several treaties signed in the wake of the US missile test in the Cuban Missile Crisis, where a scheduled missile test at a tense moment had risked exacerbating an already tense situation.
In 1971 they signed the 'Agreement on Measures to Reduce the Risk of Outbreak of Nuclear War' (because treaties relating to Nuclear Weapons always seem to have overly long titles). Article 4 stated that 'Each Party undertakes to notify the other Party in advance of any planned missile launches if such launches will extend beyond its national territory in the direction of the other Party.'
This was expanded the next year with the 'Agreement Between the Government of The United States of America and the Government of The Union of Soviet Socialist Republics on the Prevention of Incidents On and Over the High Seas', which, whilst not directly about missile tests, meant that both sides would refrain from launching anything in the vicinity of the others ships as a practice exercise.
The 1987 'Agreement Between The United States of America and The Union of Soviet Socialist Republics on the Establishment of Nuclear Risk Reduction Centers' provided special channels for communicating such information rather than using standard diplomatic protocols, as had been used prior to this.
Finally, the 1988 'AGREEMENT BETWEEN THE UNITED STATES OF AMERICA AND THE UNION OF SOVIET SOCIALIST REPUBLICS ON NOTIFICATIONS OF LAUNCHES OF INTERCONTINENTAL BALLISTIC MISSILES AND SUBMARINE-LAUNCHED BALLISTIC MISSILES' formalised the structures of communications even further, and included provisions for notification of the launch site and 24 hours notice before launching.
Sorry that its a bit on the dull side of Cold War history, but much of the negotiations over the nuclear treaties were largely multi-year sessions of unproductive waffle, with the occasional breakthrough.
Sources of the Agreements
https://fas.org/nuke/control/launch/text/balist1.htm