Backport CVE-2021-28153 symlink fix to GLib 2.58

Debian 10 contains GLib 2.58 and is supported for a bit more than 1 more year, so I need to backport the CVE-2021-28153 symlink attack fix to that version. Similar to !2000 (merged), I'd like to do this upstream.

I do not intend this to imply any particular upstream support for GLib 2.58, and in particular I don't plan to make any new GLib 2.58.x releases.

• glocalfileoutputstream: Fix a typo in a comment

  From: @pwithnall

  Trivial cherry-pick of 78420a75

• tests: Stop using g_test_bug_base() in file tests

  From: @pwithnall

  Since a following commit is going to add a new test which references Gitlab, so it’s best to move the URI bases inside the test cases.

  Backported to GLib 2.58 by Simon McVittie.

  Signed-off-by: @pwithnall
  (cherry-picked from commit 32d3d02a)
  [GLib 2.58.x did not allow g_test_bug() without g_test_bug_base(),
  so use an empty string as the base]
  Signed-off-by: @smcv

• glocalfileoutputstream: Factor out a flag check

  From: @pwithnall

  Cherry-pick of ce0eb088, applies after resolving simple conflicts (an adjacent line changed).

• glocalfileoutputstream: Fix CREATE_REPLACE_DESTINATION with symlinks

  From: @pwithnall

  Resolves CVE-2021-28153 (#2325 (closed)). Backported to GLib 2.58 by Simon McVittie.

  Signed-off-by: @pwithnall
  (cherry-picked from commit 317b3b58)
  [Backport to 2.58.x: replace g_local_file_fstat with fstat]
  [Backport to 2.58.x: replace g_local_file_lstat with lstat]
  [Backport to 2.58.x: replace _g_stat_mode with direct access to st_mode]
  [Backport to 2.58.x: don't call g_test_summary()]
  Signed-off-by: @smcv

• glocalfileoutputstream: Add a missing O_CLOEXEC flag to replace()

  From: @pwithnall

  Trivial cherry-pick of 6c643926