tests: Switch sys/kern/sysctl_security_jail_children to execenv=jail

Kyua skips tests based on the jail execution environment if a system is
built WITHOUT_JAIL. Thus, the test case does not need to handle it.

Reviewed by:	markj
Approved by:	markj (mentor)
Differential Revision:	https://reviews.freebsd.org/D47105

tests/sys/kern/Makefile

@@ -63,7 +63,6 @@ TEST_METADATA.sonewconn_overflow+=	required_user="root"
 TEST_METADATA.sonewconn_overflow+=	is_exclusive="true"
 ATF_TESTS_SH+=	sendfile_test
 ATF_TESTS_SH+=	sysctl_security_jail_children
-TEST_METADATA.sysctl_security_jail_children+=	is_exclusive="true"
 
 ${PACKAGE}FILES+=	sonewconn_overflow.py
 ${PACKAGE}FILESMODE_sonewconn_overflow.py=0555

tests/sys/kern/sysctl_security_jail_children.sh

@@ -24,21 +24,15 @@
 # OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 # SUCH DAMAGE.
 
-#
-# Even being is_exclusive="true" this test does not expect a host to spawn
-# other jails during the test execution.
-#
 atf_test_case "max_cur" "cleanup"
 max_cur_head()
 {
 	atf_set descr 'Test maximum and current number of child jails'
 	atf_set require.user root
+	atf_set execenv jail
 }
 max_cur_body()
 {
-	if ! which -s jail; then
-		atf_skip "This test requires jail"
-	fi
 	origin_max=$(sysctl -n security.jail.children.max)
 	origin_cur=$(sysctl -n security.jail.children.cur)