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_cert_macros.py b/tests/scripts/generate_test_cert_macros.py
index 1472370..07c5b7d 100755
--- a/tests/scripts/generate_test_cert_macros.py
+++ b/tests/scripts/generate_test_cert_macros.py
@@ -72,7 +72,8 @@
     """
     template_loader = jinja2.FileSystemLoader(DATA_FILES_PATH)
     template_env = jinja2.Environment(
-        loader=template_loader, lstrip_blocks=True, trim_blocks=True)
+        loader=template_loader, lstrip_blocks=True, trim_blocks=True,
+        keep_trailing_newline=True)
 
     def read_as_c_array(filename):
         with open(filename, 'rb') as f: