Merge bitcoin/bitcoin#21873: test: minor fixes & improvements for files linter test

2227fc4e6203064b14e99bcf453601bd263a0196 test: minor fixes & improvements for files linter test (windsok)

Pull request description:

  Couple of minor fixes & improvements for files linter test added in #21740

  - Use a context manager when opening files, so that files are closed are we are done with them

  - Use the `-z` flag when shelling out to `git ls-files` so that we can catch newlines and other weird control characters in filenames.

  From the `git ls-files` manpage:
  ```
  -z \0 line termination on output and do not quote filenames. See OUTPUT below for more information.

  Without the -z option, pathnames with "unusual" characters are quoted as explained for the configuration variable
  core.quotePath (see git-config(1)). Using -z the filename is output verbatim and the line is terminated by a NUL byte.
  ```

ACKs for top commit:
  MarcoFalke:
    cr ACK 2227fc4e6203064b14e99bcf453601bd263a0196
  practicalswift:
    cr ACK 2227fc4e6203064b14e99bcf453601bd263a0196: patch looks correct

Tree-SHA512: af059a805f4a7614162de85dea856052a45ab531895cb0431087e7fc9e037513fa7501bb5eb2fe43238adf5f09e77712ebdbb15b1486983359ad3661a3da0c60

0 files changed, 0 insertions, 0 deletions