[9.20] fix: test: Silence warning initialising compressed

The string literal initialalising compressed was too big for the
array as it has an unwanted NUL terminator.  This is allowed for
in C for historical reasons but produces a warning with some
compilers.  Adjust the declaration to include the NUL and adjust
the users to pass in an adjusted size which excludes the NUL rather
than sizeof(compressed).

Closes #5258

Backport of MR !10326

Merge branch 'backport-5258-avoid-warning-initialising-compresss-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10331