tests: add zfs_unshare_008_pos checking whitespace escaping

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13165

tests/zfs-tests/include/libtest.shlib

@@ -1110,6 +1110,18 @@ function datasetnonexists
 	return 0
 }
 
+# FreeBSD breaks exports(5) at whitespace and doesn't process escapes
+# Solaris just breaks
+#
+# cf. https://github.com/openzfs/zfs/pull/13165#issuecomment-1059845807
+#
+# Linux can have spaces (which are \OOO-escaped),
+# but can't have backslashes because they're parsed recursively
+function shares_can_have_whitespace
+{
+	is_linux
+}
+
 function is_shared_freebsd
 {
 	typeset fs=$1