[Bug] File mode tests for regular files pass only with specific umask values

[kevin85421]

Describe the bug

• As shown in the following code snippet, the function ensureFiles checks the file mode for both regular files and secret files.
  https://github.com/nginxinc/nginx-gateway-fabric/blob/6d4cfd7f0de32e9f98dae358cb6cec93529109a5/internal/mode/static/nginx/file/manager_test.go#L43-L47

• The function ReplaceFiles in nginx/file/manager.go creates files by internally calling os.Create, which, by default, creates files with mode 0666 (before applying umask). See the source code of os.Create for more details.

• The function writeFile changes the mode of secret files to 0640 by calling chmod, but does nothing for regular files. Hence, the check Expect(info.Mode()).To(Equal(os.FileMode(0o644))) in nginx/file/manager_test.go only passes for umask with specific values.

• In my environment, the umask value is 002. Therefore, the mode for regular files will be 0666 - 0002 = 0664, causing the unit test to fail. In the following screenshot, 420 is 0o644, and 436 is 0o664.
  

To Reproduce

• Step 1: Set umask to 002.
• Step 2: Run make unit-test.