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.
(cherry picked from commit b929e0c)

Author: ThomasWaldmann

src/borg/platformflags.py

@@ -9,4 +9,6 @@
 is_win32 = sys.platform.startswith('win32')
 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')