test: Use throwing config parser getters without fallback #35846

Currently, the called getboolean member function is not the throwing https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.getboolean, but a non-throwing member function on a dict-like proxy object.

This is confusing and brittle, because tests shouldn't silently skip when a config key is missing. Instead, tests should loudly fail, e.g. when the config key is renamed in one place, but not the other.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35846.

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