[sonic-package-manager] insert newline in /etc/sonic/generated_services.conf (#3040)

This issue is seen during upgrade from older branches to 202305 with PR
sonic-net/sonic-utilities#3037.

When sonic-package-manager installs an extension it re-generates
/etc/sonic/generated_services.conf. The file does not contain newline at
the end.

If packages are installed at runtime everything is ok, but when they are
installed at build time, /etc/sonic/generated_services.conf gets later
appended with new services:

```
echo "{{service}}" | sudo tee -a $GENERATED_SERVICE_FILE
```

This pattern is used multiple times in sonic_debian_extension.j2 script,
thus fixing it in sonic-utilities.

Signed-off-by: Stepan Blyschak <[email protected]>

Author: stepanblyschak

sonic_package_manager/service_creator/creator.py

@@ -352,6 +352,7 @@ def update_generated_services_conf_file(self, package: Package, remove=False):
         # Write to tmp file and replace the original file with it
         with tempfile.NamedTemporaryFile('w', delete=False) as tmp:
             tmp.write('\n'.join(list_of_services))
+            tmp.write('\n')
             tmp.flush()
 
             shutil.move(tmp.name, GENERATED_SERVICES_CONF_FILE)

tests/sonic_package_manager/test_service_creator.py

@@ -113,7 +113,10 @@ def read_file(name):
     assert read_file('warm-reboot_order') == 'swss teamd test syncd'
     assert read_file('fast-reboot_order') == 'teamd test swss syncd'
     assert read_file('test_reconcile') == 'test-process test-process-3'
-    assert set(read_file('generated_services.conf').split()) == set(['test.service', '[email protected]'])
+
+    generated_services_conf_content = read_file('generated_services.conf')
+    assert generated_services_conf_content.endswith('\n')
+    assert set(generated_services_conf_content.split()) == set(['test.service', '[email protected]'])
 
 
 def test_service_creator_with_timer_unit(sonic_fs, manifest, service_creator):