Work to end of August 2021

• Research of possible replacements for fbtest suite. The replacement of our choice is pytest (pytest.org).
• Development of pytest plugin to provide specific functionality needed by Firebird tests and QA process.
• Development of conversion utility to generate new pytest tests from fbtest tests. The conversion process is not able to convert all tests into working ones. Tests that were implemented in Python must be re-implemented manually (conversion only creates stubs for them). All isql-based tests are converted into working ones, but some need manual adjustments to do not fail (where originals depended on specific execution environment or were "unportable" in other ways).
• Fix of some failing tests.