Optimize Initialization of SSLContext in Unit Tests

Multiple unit tests use the StandardSSLContextService implementation class in order to support TLS communication.  As a result of this approach, each test method creates and initializes a new instance of SSLContext, which causes poor performance and intermittent timeouts.

Following the changes implemented in NIFI-8171, other unit tests should be modified to initialize an SSLContext in a method annotated with @BeforeClass and provide that SSLContext using a mock of the SSLContextService interface.