ash: fix var_leak.tests so that it actually catches the NOFORK bug

+ document the bug better

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash_test/ash-vars/var_leak.right

@@ -1,3 +1,4 @@
 should be empty: ''
+should be empty: ''
 should be not empty: 'val2'
 should be not empty: 'val3'

shell/ash_test/ash-vars/var_leak.tests

@@ -1,6 +1,11 @@
-# true is a regular builtin, varibale should not leak out of it
+# cat is an external program, variable should not leak out of it.
 # this currently fails with CONFIG_FEATURE_SH_NOFORK=y
 VAR=''
+VAR=val0 cat /dev/null
+echo "should be empty: '$VAR'"
+
+# true is a regular builtin, variable should not leak out of it.
+VAR=''
 VAR=val1 true
 echo "should be empty: '$VAR'"