88a35c8918 test: add test for new sign fn (Chris Hyunhum Cho)
Pull request description:
Follow up https://github.com/rust-bitcoin/rust-bitcoin/pull/3456.
add test to verify the signature generated from newly added `sign` fn.
ACKs for top commit:
apoelstra:
ACK 88a35c8918f48e72f032b73a233a016a1a0056b6; successfully ran local tests; sorry, testing local CI changes..
tcharding:
ACK 88a35c8918
Tree-SHA512: 8c457fbabf1ff86a9369934564874e78660f706714465731c185da16eff5c88a5e680c4373effbcc273394a5653edde9f559558917fee6e438b79bfee6f63b22