Adds tests for the locate_signed method in
GlobalLocator that match the "locate" method tests.

Note that the invalid sgid test case currently raises a
ActiveSupport::MessageVerifier::InvalidSignature. I think the intended
design is for the initializer to return nil, but I wasn't sure.

SignedGlobalId initializer could check for a valid sgid.

Removed same tests from previous location.
Merge pull request #4 from andyatkinson/small_testing_additions

Improve tests