[9.20] fix: test: Limit X-Bloat header size to 100KB

[9.20] fix: test: Limit X-Bloat header size to 100KB

Otherwise curl 8.13 rejects the line with:

    I:Check HTTP/1.1 keep-alive with truncated stream (21)
    curl: option --header: error encountered when reading a file
    curl: try 'curl --help' or 'curl --manual' for more information

Also, see https://github.com/curl/curl/pull/16572.

Closes #5249

Backport of MR !10319

Merge branch 'backport-5249-statschannel-limit-http-header-size-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10321