fix: testutil network app config setup

[aaronc]

Description

This fixes an issue uncovered in #12243 where testutil/network validators share the same app instance.

It also fixes the signature of runtime.AppBuilder.Build which requires passing back the MsgServiceRouter that runtime already created.

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[aaronc]

No need to resolve this and then pass this parameter back into Build. runtime created it in the first place

[likhita-809]

The changes from here are cleanly merged into main as a part of #12243
Closing this since there are no other new changes.