Test generation is not canceled when user tries to stop it at building phase

[tyuldashev]

Description

For big projects build phase could take noticeable amount of time if user tries to cancel test generation at the moment UBTot still proceeds with test generation.

To Reproduce

Steps to reproduce the behavior:

1. Open utbot project
2. Invoke test generation on some class, on org.utbot.examples.strings.StringExamples for example
3. Quickly click on cancel while UTBot still at building phase
   

Expected behavior

Generation of tests stopped

Actual behavior

UTBot stops (or try to stop building) but then procced with reading classes and test generation.

[Vassiliy-Kudryashov]

This issue duplicates #1100 and can be closed as obsolete (see revision 53c3b3f8)