[To dev/1.3] [Test] Exclude mock initialization from pipe worker timeouts (#18886)

[Caideyipi]

Description

Backport #18886 (f660639add95e5e4ba392a4cdc2467aaf8aea489) to dev/1.3.

dev/1.3 contains the same worker-notification tests and timed mock creation as master. In a fresh Windows CI fork, Mockito/Byte Buddy initialization can consume the test timeout before the workers start. Initialize a PipeProcessorSubtask mock in @BeforeClass so that startup happens before JUnit starts the test timers. The existing 10-second and 30-second deadlines and the 5-second admission wake-up assertion remain unchanged.

The cherry-pick applied without conflicts and adds only the original eight lines to PipeProcessorSubtaskWorkerTest.

Validation

• mvn -B -ntp -Ddevelocity.off=true spotless:apply -pl iotdb-core/datanode — passed; no formatting changes.
• mvn -B -ntp test -pl iotdb-core/datanode -am -Dtest=PipeProcessorSubtaskWorkerTest -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false -Ddevelocity.enabled=false — all 26 reactor modules built successfully from a fresh worktree; all 6 tests passed in both configured Surefire executions, with no failures, errors, or skips. Checkstyle and Spotless also passed.
• git diff origin/dev/1.3...HEAD --check — passed.

This PR has:

• been self-reviewed.
• added comments explaining the intent.
• modified existing tests to address the CI failure.