Improve robustness of chttpd_util_test

[jaydoane]

In their current form, some of these tests rely on configuration props
set with specific values in rel/overlay/etc/default.ini, which makes
them prone to breakage when those values change, or when tests run in
non-default configuration.

This change deletes all config settings in the relevant sections under
test, and then adds those under test back explicitly.

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation