Fix Python API mixin modifying the input file

Move configuration involving the input and output files to before the
run stage to allow mixins to change the file names if required

reframe_tests/tests/base_tests.py

@@ -106,7 +106,7 @@ class GprMaxBaseTest(RunOnlyRegressionTest):
 
     model = parameter()
     sourcesdir = required
-    executable = "time -p python -m gprMax --log-level 10 --hide-progress-bars"
+    executable = "time -p python -m gprMax"
 
     regression_checks = variable(typ.List[RegressionCheck], value=[])
 
@@ -159,16 +159,29 @@ class GprMaxBaseTest(RunOnlyRegressionTest):
         self.prerun_cmds.append(f"source {path_to_pyenv}")
 
     @run_after("init")
+    def set_file_paths(self):
+        self.input_file = Path(f"{self.model}.in")
+        self.output_file = Path(f"{self.model}.h5")
+
+    @run_before("run")
     def configure_test_run(self):
         """Configure gprMax commandline arguments and plot outputs
 
         Set the input and output files and add postrun commands to plot
         the outputs.
         """
-        self.input_file = f"{self.model}.in"
-        self.output_file = f"{self.model}.h5"
-        self.executable_opts = [self.input_file, "-o", self.output_file]
-        self.keep_files = [self.input_file, self.output_file]
+        input_file = str(self.input_file)
+        output_file = str(self.output_file)
+
+        self.executable_opts += [
+            input_file,
+            "-o",
+            output_file,
+            "--log-level",
+            "10",
+            "--hide-progress-bars",
+        ]
+        self.keep_files += [input_file, output_file]
 
         """
         if self.has_receiver_output:

reframe_tests/tests/mixins.py

@@ -72,11 +72,10 @@ class SnapshotMixin(GprMaxMixin):
 
 
 class PythonApiMixin(GprMaxMixin):
-    executable = "time -p python"
-
     @run_after("setup")
     def set_python_input_file(self):
         """Input files for API tests will be python files"""
+        self.executable = "time -p python"
         self.input_file = self.input_file.with_suffix(".py")