Make the tool generic: load list of testcases to run from .nxdtest file
and process the correspondingly. See added top-level documentation for
details.

Amends e9cc7e07 (Add minimal packaging setup)

So that nxdtest could be installed as egg via pip or buildout.

We will use it as the base for nxdtest.
Original history:

kirr/neo@51b18490
kirr/neo@f67c147d
kirr/neo@0e28fbb8

We send output from tested process to master. We also print it to
stdout,stderr so it appears in testnode logs.

However till now it was like, whole output first read, and only then
emitted to log as a whole, thus not allowing to oversee current test
progress by watching testnode log tail.

Fix it by implementing the teeing process manually.

Some draft history related to this patch:

	lab.nexedi.com/kirr/neo/commit/aa370ca3        fixup! X neotest/runTestSuite: Tee tested process stdout,stderr to testnode logs incrementally
	lab.nexedi.com/kirr/neo/commit/096550b1        fixup! X neotest/runTestSuite: Tee tested process stdout,stderr to testnode logs incrementally
	lab.nexedi.com/kirr/neo/commit/63956f43        fixup! X neotest/runTestSuite: Tee tested process stdout,stderr to testnode logs incrementally
	lab.nexedi.com/kirr/neo/commit/b9819d0e        X neotest/runTestSuite: Tee tested process stdout,stderr to testnode logs incrementally

The wrapper runs `neotest bench-local` in neotest SlapOS instance:

	https://lab.nexedi.com/nexedi/slapos/blob/ab8705f4/software/neotest/instance.cfg.in

Extracted from slapos!282

Nxdtest is conceived as a tox-like tool to run tests for a project under
Nexedi testing infrastructure.