Existing test framework should be pre-selected at the very first start

[Vassiliy-Kudryashov]

Description

In case 'current' framework and 'default' framework both not installed we pre-select the installed one (if any)

Fixes #155

Type of Change

• Usability bug fix (non-breaking small changes)

How Has This Been Tested?

Manual Scenario

1. Generate some tests with JUnit 4 or TestNG
2. Shutdown IDE, delete .idea\utbot-settings.xml
3. Start again, open "Gennerate..." dialog
4. Existing test framework should be selected (instead of 'default' JUnit 5)
5. Remove JUnit 4 or/and TestTG library from project dependency (Go to Project Structure > Modules > Depencencies > Remove)
6. Open "Generate..." dialog, now JUnit 5 is pre-selected again as default.

Checklist (remove irrelevant options):

• The change followed the style guidelines of the UTBot project
• Self-review of the code is passed
• No new warnings