hex_psbt was added as a macro so that a panic would reveal the line
number of the failing test by expanding the macro at the test location.
However, a stack trace can be used to reveal the test that caused the
failure using RUST_BACKTRACE=1. Furthermore, the track_caller macro is
added to the helper methods which will reveal the line number of the
calling function (the offending test). More detailed information for
debugging has been added to hex_psbt() so that the offending string
will be included in the panic message.
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