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/data_files/test_certs.h.jinja2 b/tests/data_files/test_certs.h.jinja2
index 4a64b3a..f2657d8 100644
--- a/tests/data_files/test_certs.h.jinja2
+++ b/tests/data_files/test_certs.h.jinja2
@@ -40,3 +40,4 @@
     {% endif %}
 
 {% endfor %}
+/* End of generated file */