Support system tests with arbitrary many *.lobster files

Add `README.md` to explain how system tests work.

tests-system/README.md

@@ -0,0 +1,41 @@
+The script `run_tool_tests.py` executes system tests.
+It can execute system tests for all lobster tools.
+
+The one and only command line argument to the script must be the name of the tool under
+test.
+For example, to run system tests for `lobster-trlc`, the tool must be started like this:
+```
+> python3 run_tool_tests.py lobster-trlc
+```
+
+System test cases always consist of the following folder structure:
+```
+test-case/
+├─ input/
+│  ├─ args.txt
+expected-output/
+├─ exit-code.txt
+├─ stderr.txt
+├─ stdout.txt
+
+```
+
+The target `make system-tests` in the root `~Makefile` sets the current working
+directory of the tool under test to the `input` folder.
+
+The files must contain the following piece of information:
+- `args.txt`:
+  This file shall contain the command line arguments that must be used to start the tool
+  under test. Each line represents a separate argument.
+- `exit-code.txt`: This file shall contain the expected exit code of the tool under test.
+- `stderr.txt`: This file shall contain the expected standard error stream of the tool
+  under test.
+- `stdout.txt`: This file shall contain the expected standard output stream of the tool
+  under test.
+
+Any additional files needed by the tool under test must be located in the `input`
+folder.
+
+The expected values will all be compared against their actual values, and test cases
+only count as "passed" if the values match.
+

tests-system/run_tool_tests.py

@@ -51,9 +51,7 @@ def _get_expected_lobster_output_file_name(self) -> str:
 
         Note: The system test must always be prepared such that the tool under test
         generates the lobster file in the "input" folder. Other test setups are not
-        supported. Furthermore, the tool under test must generate exactly one output
-        file. Tools like 'lobster-cpptest' are able to generate multiple files. Testing
-        that is currently not supported.
+        supported.
         """
         for dir_entry in scandir(self.get_expected_output_path()):
             if (not dir_entry.is_dir()) and dir_entry.name.endswith(".lobster"):