Use file_name parameter in walk_ssl_opt_sh()

Remove hardcoded file_name and use the parameter provided in the
function.

Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
diff --git a/tests/scripts/check_test_cases.py b/tests/scripts/check_test_cases.py
index 96e3839..0ca0def 100755
--- a/tests/scripts/check_test_cases.py
+++ b/tests/scripts/check_test_cases.py
@@ -102,7 +102,7 @@
     def walk_ssl_opt_sh(self, file_name):
         """Iterate over the test cases in ssl-opt.sh or a file with a similar format."""
         descriptions = self.new_per_file_state() # pylint: disable=assignment-from-none
-        listed = subprocess.run(['tests/ssl-opt.sh', '-l'],
+        listed = subprocess.run([f'{file_name}', '-l'],
                                  capture_output=True)
         listed = set(map(lambda x: x.rstrip(), listed.stdout.splitlines()))
         for description in listed: