Inject absolute paths into "expected output" files (#157)

Add feature to the system test framework to generate absolute paths in
output files based on template files.

It works like this:
If a folder called `expected-output-template` exists, then
1. it takes each file from that folder
2. creates a copy in folder `expected-output`
3. and replaces the string `TESTS-SYSTEM` with an absolute path to the
`tests-system` folder.

This way it is possible to compare *.lobster files that contain absolute
paths against their expected content. `lobster-cpptest` needs this
feature.

tests-system/run_tool_tests.py

@@ -1,4 +1,5 @@
 import sys
+from posixpath import normpath
 from os import scandir, DirEntry, remove
 from os.path import basename, dirname, join
 from pathlib import Path
@@ -31,6 +32,11 @@ def __init__(self, test_case_path: str):
         self._expected_lobster_output_file_name = \
             self._get_expected_lobster_output_file_name()
 
+    @property
+    def test_case_path(self) -> str:
+        """Returns the path to the directory which contains the test case"""
+        return self._test_case_path
+
     @property
     def expected_lobster_output_file_name(self) -> str:
         return self._expected_lobster_output_file_name
@@ -185,18 +191,27 @@ def _compare_results(setup: TestSetup, completed_process: CompletedProcess):
     with open(expected, "r", encoding="UTF-8") as expected_lobster_file:
         try:
             with open(actual, "r", encoding="UTF-8") as actual_lobster_file:
-                # Note: we replace Windows-like slashes \\ with one / in order to be
-                # able to compare the actual output on all OS against the expected
-                # output on Linux
-                assert actual_lobster_file.read().replace("\\\\", "/") \
-                    == expected_lobster_file.read(), \
+                # Before comparing the actual text with the expected text, we do the
+                # following replacements:
+                # a) Replace Windows-like slashes \\ with / in order to be able to
+                #    compare the actual output on all OS against the expected output on
+                #    Linux
+                # b) Replace the fixed string TEST_CASE_PATH with the absolute path to
+                #    the current test case directory. This is necessary for tools like
+                #    lobster-cpptest which write absolute paths into their *.lobster
+                #    output files.
+                modified_actual = actual_lobster_file.read().replace("\\\\", "/")
+                modified_expected = expected_lobster_file.read().replace(
+                    "TEST_CASE_PATH", setup.test_case_path
+                )
+                assert modified_actual == modified_expected, \
                     "Actual *.lobster file differs from expectation!"
         except FileNotFoundError as ex:
             assert True, f"File {ex.filename} was not generated by the tool under test!"
 
 
 def _get_directories(
-        start_directory: str,
+        start_directory: Path,
         startswith: Optional[str] = None,
     ) -> Iterator[DirEntry]:
     """Returns DirEntry instances for each subdirectory found in the given start
@@ -225,7 +240,7 @@ def _delete_generated_files(setup: TestSetup):
     remove(generated)
 
 
-def _run_tests(directory: str, tool: str) -> int:
+def _run_tests(directory: Path, tool: str) -> int:
     """Runs all system tests in the given folder for the specified tool.
 
     :param directory: the path to the directory containing all test cases
@@ -264,7 +279,7 @@ def _get_tool(test_dir: str) -> str:
     to the tool name
     :param test_dir: The path containing the requirements-based tests
     """
-    return join("../../../../../", basename(test_dir))
+    return normpath(Path(join("../", basename(test_dir))).absolute())
 
 
 if __name__ == "__main__":