[Unit Tests] Fix lag after Unit Tests finish, fix flakiness in Sampling test

[jj22ee]

Issue #, if available:

• npm run test command stalls for 20s after all tests pass (or fail), discovered caused by Concurrency test, likely due to exportTimeoutMillis default value being set in AwsBatchUnsampledSpanProcessor
• Flaky test case testSomeReservoir that may fail when sampling targets are not refreshed in time for the unit tests (due to the test being reliant on setTimeout which doesn't guarantee exact time of execution).
  • https://github.com/aws-observability/aws-otel-js-instrumentation/actions/runs/11023689994/job/30615442620#step:7:560

Description of changes:

• Add short exportTimeoutMillis value for AwsBatchUnsampledSpanProcessor config in Concurrency test
• Update testSomeReservoir to properly cleanup the fake clock, provide more time for Sampling targets to refresh on time for the unit test run.

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