The PulsarTemplate recently replaced its list of ProducerInterceptors
with a list of ProducerBuilderCustomizers that customize the builder by
adding each interceptor to the builder. The PulsarAutoConfigurationTests
previosuly relied on the previous field. This commit adjusts the tests
to instead use the Customizers testing utility to verify the
interceptors.
(cherry picked from commit 9c054a021fc34728a68a835a5c2e421f3e11df9f)
See gh-39946