WIP: Separate python framwork of tests and actual tests

This work separates the python test framework (all the code that instantiates nodes and clients, makes rpc calls, and runs commands) and the tests that use this framework to run. The framework is moved to vendors/ (alongside flextesa-lib which has a similar role). The tests themselves remain in tests_python.

Beyond just moving the test framework to be alongside the other one (flextesa), this MR makes it easier for other projects to use the test framework. Specifically they are able to easily copy the framework without importing all the tests along with it. (At least @pietro and @arvidnl have expressed interest in doing so.)

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Possible future works (which, if we decide to do them, would be in separate MRs):

• release the test framework
• move the tests so that they mirror the structure of flextesa-based tests (i.e., move them to src/bin-<smthg> and have src/bin-<smthg>/dune define aliases for launching the tests so that dune runtest Just Works™)