generate_test_[keys/cert_macros]: minor fixes

- remove new line at beginning of test_keys.h
- add footer at the end of both generated files

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 335e848..177850e 100755
--- a/tests/scripts/generate_test_keys.py
+++ b/tests/scripts/generate_test_keys.py
@@ -92,7 +92,7 @@
 
 def write_output_file(output_file_name: str, arrays: List[str], look_up_table: List[str]):
     with open(output_file_name, 'wt') as output:
-        output.write("""
+        output.write("""\
 /*********************************************************************************
  * This file was automatically generated from tests/scripts/generate_test_keys.py.
  * Please do not edit it manually.
@@ -112,6 +112,8 @@
 struct predefined_key_element predefined_keys[] = {{
 {}
 }};
+
+/* End of generated file */
 """.format("\n".join(look_up_table)))