[test] README

test/README.md

+### Running tests
+```
+pytest
+```
+
+### How it works?
+1. pytest generates combinations of test input (code in [conftest.py][conftest.py])
+2. for each combination of inputs: (code in [test_runner.py][test_runner.py])
+  1. pytest generates a [robin][robin] input file
+  2. pytest executes [robin][robin] on the generated file
+
+### Running a specific test
+You can therefore directly rerun a test (a failing one for example):
+```
+robin test-instances/FAILING-TEST.yaml
+```
+
+You can also run batsim and batsched in different terminals:
+```
+./test-out/FAILING-TEST/cmd/batsim.bash
+./test-out/FAILING-TEST/cmd/batsim.bash
+```
+
+[conftest.py]: ./conftest.py
+[test_runner.py]: ./test_runner.py
+[robin]: https://framagit.org/batsim/batexpe