Reword message for initial stage of test generation

[Vassiliy-Kudryashov]

Description

Fixes #2516

"Reading classes" message is pretty misleading because it's stage of initialization: starting engine, creating application context, creating test generator. In my environment starting engine takes ~3sec and creating test generator takes ~10sec. It requires further profiling.

Manual tests

Start tests generation.
For the first stage we display text "Generate tests: starting engine" then iteration through classes (with messages "Generate test cases for class XXX").

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