fix "make generated_files" for test_keys.h and test_certs.h

This also add the check in tests/scripts/check-generated-files.sh

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/scripts/generate_test_keys.py b/tests/scripts/generate_test_keys.py
index dbdb960..85ff918 100755
--- a/tests/scripts/generate_test_keys.py
+++ b/tests/scripts/generate_test_keys.py
@@ -99,9 +99,9 @@
     args = argparser.parse_args()
 
     output_file = args.output
-    # Remove output file if already existing.
+    # If the output file already exists, then we can quit (successfully)
     if os.path.exists(output_file):
-        os.remove(output_file)
+        return
 
     output_file = open(output_file, 'at')
     output_file.write(