Allow to test run flows with subflow activities

[fcastill]

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[x] feature request
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Current behavior (how does the issue manifest): Current test runner doesn't support subflows.

Expected behavior: To be able to run flows that reference other flows (subflows)

Minimal steps to reproduce the problem (not required if feature enhancement): Test run any flows that contains subflow tasks

What is the motivation / use case for changing the behavior? Enabling the Web UI to test run flows with subflows.

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): v0.5.5