gnat_compare test driver: handle more fields for test configuration

Project paths, variables, as well as preprocessor data are now handled
by the gnat-compare driver (same as for name-resolution).

testsuite/drivers/gnat_compare_driver.py

@@ -1,4 +1,6 @@
+from __future__ import annotations
 from drivers.base_driver import BaseDriver
+import os.path
 
 
 class GNATCompareDriver(BaseDriver):
@@ -12,15 +14,56 @@ class GNATCompareDriver(BaseDriver):
     * ``project_file`` (mandatory): A filename for a project file, used to
       create a project unit provider.
 
+    * ``project_path``: List of directory names (relative to the testsuite root
+      directory) to add to the project files lookup path
+      (``GPR_PROJECT_PATH``).
+
+    * ``project_vars``: Name/value mapping for project files external
+      variables.
+
+    * ``preprocessor_data_file``: Filename for the preprocessor data file.
+
+    * ``preprocessor_path``: List of directories where preprocessor data files
+      can be found.
+
     * ``comparisons``: Select what differences between GNAT's xrefs and
       Libadalang's to report.
     """
 
     def run(self):
+        env: dict[str, str] = {}
+        """
+        Additional environment variables for "nameres".
+        """
+
+        # Path for project files: make directories from the "project_path" key
+        # prioritary and append existing paths from the environment.
+        env["GPR_PROJECT_PATH"] = os.path.pathsep.join(
+            [
+                os.path.join(self.env.root_dir, p)
+                for p in self.test_env.get("project_path", [])
+            ]
+            + os.environ.get("GPR_PROJECT_PATH", "").split(os.path.pathsep)
+        )
+
         project_file = self.test_env['project_file']
         comparisons = self.test_env.get('comparisons')
 
         argv = ['gnat_compare', '-P' + project_file]
         if comparisons:
             argv.append('-d{}'.format(comparisons))
-        self.run_and_check(argv, memcheck=True)
+
+        for name, value in sorted(
+            self.test_env.get("project_vars", {}).items()
+        ):
+            argv.append(f"-X{name}={value}")
+
+        # Preprocessor handling
+        preprocessor_data_file = self.test_env.get("preprocessor_data_file",
+                                                   None)
+        preprocessor_path = self.test_env.get("preprocessor_path", [])
+        if preprocessor_data_file:
+            argv.append(f"--preprocessor-data-file={preprocessor_data_file}")
+            argv.extend(f"--preprocessor-path={d}" for d in preprocessor_path)
+
+        self.run_and_check(argv, memcheck=True, env=env)