DERBY-6716: Minor fixes in SystemPrivilegesPermissionTest

Fix typos to improve test coverage:

- check both "p1 implies p2" and "p2 implies p1" in
  assertEquivalentPermissions(), instead of checking "p1 implies p2"
  twice.

- remove a stray if statement that disables an assert in situations
  where it's expected to succeed.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1620828 13f79535-47bb-0310-9956-ffa450edef68