test/crimson: trade operator<<() for fmt::format<>()

test/crimson: trade operator<<() for fmt::format<>()

so we are able to format Payload using fmtlib v9. before fmtlib v9,
fmtlib is able to fall back to the operator<<() to format the the
types with the operator<<() defined. but after fmtlib v9, we need to
explicitly define them for accessing the specialized formatter.

now that we have specialized fmt::format<> for Payload, and the only
consumer of operator<<() is the fmtlib, we can safely drop the latter.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>