docs(tests): initial unit-tests readme

Signed-off-by: Lorenzo Fontana <lo@linux.com>

tests/README.md

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+# Falco unit tests
+
+This folder contains the unit-tests suite for Falco.
+The framework we use for unit-tests is [Catch2](https://github.com/catchorg/Catch2), while the one we use for mocking is [FakeIt](https://github.com/eranpeer/FakeIt).
+
+
+## How to write tests
+
+When you want to test a new file or test a non tested file, remember four steps:
+
+- The folder structure here is the same as the one in the `userspace` folder, so `userspace/engine` becomes `tests/engine`.
+- We call test files with this format `test_<original-file-name>.cpp`
+- Update the `CMakeLists.txt` file to include your file in `FALCO_TESTS_SOURCES` and change the `FALCO_TESTED_LIBRARIES` accordingly. You might aslo need to add dependencies, in that case, look at `target_link_libraries` and `target_include_directories`
+- If you are unsure on how to write tests, refer to our existing tests in this folder and to the [Catch2](https://github.com/catchorg/Catch2/tree/master/docs) documentation.
+
+## How to execute tests
+
+The suite can be configured with `cmake` and run with `make`.
+
+
+In the root folder of Falco, after creating the build directory:
+
+```bash
+cd falco
+mkdir build
+cd build
+```
+
+You can prepare the tests with:
+
+```
+cmake ..
+```
+
+Optionally, you can customize the test suite by passing custom arguments like the examples below:
+
+**filter all tests containing the word ctor**
+
+```bash
+cmake -DFALCO_TESTS_ARGUMENTS:STRING="-R ctor" ..
+```
+
+**verbose execution**
+
+```bash
+cmake -DFALCO_TESTS_ARGUMENTS:STRING="-V" ..
+```
+
+
+To see a list of all the custom arguments you may pass, execute `ctest --help` in your terminal.
+
+
+Once you are ready, you can run your configuration with:
+
+```bash
+make tests
+```
+