Suppress logback status logging in tests

[mrdziuban]

Fixes #5942

digital-asset/canton#535 moved community/util-observability/src/test/resources/logback-test.xml to base/testing-utils/src/main/resources/logback-test.xml, resulting in logback-test.xml being bundled in the com.daml::testing-utils JAR file. This conflicts with the logback-test.xml in this repo (at canton/community/util-observability/src/test/resources/logback-test.xml) and causes logback to log warnings:

Resource [logback-test.xml] occurs multiple times on the classpath.
Resource [logback-test.xml] occurs at [jar:file:/home/matt/.cache/coursier/v1/https/repo1.maven.org/maven2/com/daml/testing-utils_2.13/3.5.3/testing-utils_2.13-3.5.3.jar!/logback-test.xml]
Resource [logback-test.xml] occurs at [file:/home/matt/Projects/splice/canton/community/util-observability/target/scala-2.13/test-classes/logback-test.xml]

This adds a statusListener of ch.qos.logback.core.status.NopStatusListener to the config to suppress the logs, but it doesn't actually fix the issue of there being two logback-test.xml files on the classpath.

If that's not a sufficient fix, there are a couple other alternatives:

1. Specify the correct config file via Test / javaOptions += "-Dlogback.configurationFile=.../logback-test.xml" -- this would need to be set on every affected sbt project
2. Move logback-test.xml in the canton repo to src/test/resources (instead of src/main/resources) so it's not bundled in the JAR -- this would then require an update to com.daml::testing-utils once the upstream change is released

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