Correct the top warnings mask for 5.6 in t/07-context_info.t

Correct the top warnings mask for 5.6 in t/07-context_info.t

It changes depending on whether the test is called with -w or not, which
itself depends on the version of Test::Harness used. To add to the
confusion, perl 5.6.1 comes without Test::More, so installing it pulls a
recent version of Test::Harness that does no longer enforces -w.