[CI] SetPriorityActionTests has invalid test for testNonPositivePriority

[markharwood]

Zero is a valid value according to the class under test but testNonPositivePriority is expecting an exception.

Build failure: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+g1gc/290/console

Reproduce with:

./gradlew :x-pack:plugin:core:unitTest -Dtests.seed=9FD1CC17D355FD0D -Dtests.class=org.elasticsearch.xpack.core.indexlifecycle.SetPriorityActionTests -Dtests.method="testNonPositivePriority" -Dtests.security.manager=true -Dtests.jvm.argline="-XX:-UseConcMarkSweepGC -XX:+UseG1GC" -Dtests.locale=es-DO -Dtests.timezone=Asia/Katmandu -Dcompiler.java=11 -Druntime.java=8

[elasticmachine]

Pinging @elastic/es-core-features

[markharwood]

Muted in 468bae2

[costin]

Cherry-picked the commit for 6.x in 1b85b19