Fix bug when restoring tests using ReceiverMixin

- 'output_file' is not stored in the run report by reframe. Therefore,
  trying to access 'test_dependency.output_file' where 'test_dependency'
  is a restored test case was returning None and causing an error.
- Instead, created a new function to build an output file path that
  can be reused by the ReceiverMixin class.

reframe_tests/tests/base_tests.py

@@ -248,6 +248,20 @@ class GprMaxBaseTest(RunOnlyRegressionTest):
             # Set the matplotlib cache to the work filesystem
             self.env_vars["MPLCONFIGDIR"] = "${HOME/home/work}/.config/matplotlib"
 
+    def build_output_file_path(self, filename: str) -> Path:
+        """Build output file Path object from filename.
+
+        Using a function to build this allows mixins to reuse or
+        override it if needed.
+
+        Args:
+            filename: Name of output file with no file extension.
+
+        Returns:
+            Path: Output file Path object
+        """
+        return Path(f"{filename}.h5")
+
     @run_after("init")
     def set_file_paths(self):
         """Set default test input and output files.
@@ -256,7 +270,7 @@ class GprMaxBaseTest(RunOnlyRegressionTest):
         later in the pipeline.
         """
         self.input_file = Path(f"{self.model}.in")
-        self.output_file = Path(f"{self.model}.h5")
+        self.output_file = self.build_output_file_path(self.model)
 
     @run_before("run")
     def configure_test_run(self):

reframe_tests/tests/mixins.py

@@ -35,7 +35,8 @@ class ReceiverMixin(GprMaxMixin):
     def add_receiver_regression_checks(self):
         test_dependency = self.get_test_dependency()
         if test_dependency is not None:
-            reference_file = self.build_reference_filepath(test_dependency.output_file)
+            output_file = self.build_output_file_path(test_dependency.model)
+            reference_file = self.build_reference_filepath(output_file)
         else:
             reference_file = self.build_reference_filepath(self.output_file)