Very minor code cleanup (whitespace changes only)

[emmenlau]

This commit makes very minor code cleanup improvements to file lib/cpp/test/concurrency/Tests.cpp, and adds a missing include.

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[emmenlau]

No build problems or errors are related to this change. Failing tests already failed before:

• https://ci.appveyor.com/project/ApacheSoftwareFoundation/thrift/builds/48258441/job/usyl6q6av8ks9rib
• https://ci.appveyor.com/project/ApacheSoftwareFoundation/thrift/builds/48258441/job/2jvgkyk39gv9rxwb