Set 'task.partitioned-writer-count' property to processor count in testExplicitPropertyMappings test

[sanchitkashyap]

In the current state, the test can sometimes fail depending on the number of cores of the machine in which the test is running.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Sanchit Kashyap.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[sopel39]

Thanks
@sanchitkashyap please also file CLA