freebsd/netbsd: skip test_hard_link_deletion_and_replacement, #9147, #9153

The test fails on these platforms.

I could not find the root cause of this issue, but it is likely a minor
problem with ctime and doesn't affect borg usage much.

So I rather like to have CI on freebsd/netbsd not failing because of this.

Also: add is_netbsd and is_openbsd to platformflags.

src/borg/platformflags.py

@@ -11,4 +11,6 @@ is_cygwin = sys.platform.startswith("cygwin")
 
 is_linux = sys.platform.startswith("linux")
 is_freebsd = sys.platform.startswith("freebsd")
+is_netbsd = sys.platform.startswith("netbsd")
+is_openbsd = sys.platform.startswith("openbsd")
 is_darwin = sys.platform.startswith("darwin")

src/borg/testsuite/archiver/diff_cmd_test.py

@@ -7,7 +7,7 @@ import pytest
 
 from ...constants import *  # NOQA
 from .. import are_symlinks_supported, are_hardlinks_supported, granularity_sleep
-from ...platformflags import is_win32
+from ...platformflags import is_win32, is_freebsd, is_netbsd
 from . import (
     cmd,
     create_regular_file,
@@ -426,7 +426,10 @@ def test_sort_by_all_keys_with_directions(archivers, request, sort_key):
         assert seen_paths == expected_paths
 
 
-@pytest.mark.skipif(not are_hardlinks_supported(), reason="hardlinks not supported")
+@pytest.mark.skipif(
+    not are_hardlinks_supported() or is_freebsd or is_netbsd,
+    reason="hardlinks not supported or test failing on freebsd and netbsd",
+)
 def test_hard_link_deletion_and_replacement(archivers, request):
     archiver = request.getfixturevalue(archivers)