Support use of policies.json

At Tails, we are currently having trouble porting our customizations for Tor Browser 9. What would really help is the functionality of policies.json. I see that you add support for using policies.json with Tor Browser in legacy/trac#29445 (moved), but then reverted that fix in legacy/trac#29916 (moved).

In legacy/trac#29916 (moved)#comment:7 and legacy/trac#29916 (moved)#comment:8, you state that setting browser.policies.testing.disallowEnterprise=true should be enough to fix legacy/trac#29916 (moved). So would it be possible to reintroduce the fix for legacy/trac#29445 (moved), so that we can set browser.policies.testing.disallowEnterprise=false in Tails and make use of the policies.json?