App test doesn't free server port.

[gladcow]

Bug Description
Ports 3939 and 4040 a busy after tests/app_test is completed.

Impacts

• [*] API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• [*] Deployment (the deployment process after building such as Docker, Gitlab, etc.)
• [*] P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• [*] UX (the User Experience)
• Other (please add below)

Steps To Reproduce
Start tests\app_test twice, one after other. Second run is failed due to busy ports

Expected Behavior
Both test starts should be successful

Host Environment

• Host OS: Ubuntu 18.04 LTS
• Host Physical RAM 16GB
• SSL: OpenSSL 1.1.1 11 Sep 2018
• Boost: 1.67

[bobinson]

fixed by #154