Configuration reader should enforce testnet initial balances

[kantai]

Connecting to the Stacks testnets requires having the same initial balances configured for the genesis block as the rest of the network. Otherwise, the initialized node will operate as a separate fork.

Currently, this requires specifying those ustx_balance entries explicitly in the config file (even if you don't intend to change them).

Two things would improve the experience around configuring a testnet node:

1. Making those entries default when the configuration file otherwise specifies the node is a testnet node.
2. If a node is a testnet node and the configuration file specifies ustx_balance entries that are incorrect, then either (a) exit with an error code or (b) issue a warning.

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