66fe7b1a98c03f690dcf60d359baac124658aeae test: added test for upgradewallet RPC (Harris)
Pull request description:
This PR adds tests for the newly merged *upgradewallet* RPC.
Additionally, it expands `test_framework/util.py` by adding the function `adjust_bitcoin_conf_for_pre_17` to support nodes that don't parse configuration sections.
This test uses two older node versions, v0.15.2 and v0.16.3, to create older wallet versions to be used by `upgradewallet`.
Fixes https://github.com/bitcoin/bitcoin/issues/18767
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Tree-SHA512: bb72ff1e829e2c3954386cc308842820ef0828a4fbb754202b225a8748f92d4dcc5ec77fb146bfd5484a5c2f29ce95adf9f3fb4483437088ff3ea4a8d2c442c1
