Add `selftest` target to Scons and run it in CI

[adrianopol]

Now, we have a new command to run self-tests for building scripts:

python scripts/scons_helpers/build-3rdparty.py --self-test

This should be added as a new step in CI pipeline and be called as scons selftest.

Acceptance criteria

• [x] scons selftest runs all self-tests (there is only file currently); GetPythonExecutable() should be used to call a correct Python interpreter executable
• [x] this action is run on every source code change (once for each pipeline)
• [ ] docs updated (scons options and developer cookbook pages)

[gavv]

Let's add a scons target that invokes that command, say scons selftest, and then invoke this target from CI.

Notes:

• it should use GetPythonExecutable, not just python
• it should be added to documentation (scons options and developer cookbook pages)

[gavv]

"doc-tests" sounds a bit misleading, because other doc* targets are related to documentation.

What do you think about "selftest" I suggested above?

[adrianopol]

agree, let's call it selftest

[gavv]

Landed.