Include utbot-executor and fix bugs

[tamarinvs19]

Description

• Include utbot_executor in special module into UTBot repo
• Fix problem with relative path to output directory in cli
• Update readme and tests

How to test

Automated tests

Gradle task pytest/runTests runs tests for utbot-executor.

Self-check list

Check off the item if the statement is true. Hint: [x] is a marked item.

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• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.