gd: Include URI in comparisons between GDataGDPhoneNumbers

This means two numbers will compare equal if their URIs are
non-NULL and equal. This is a slight behaviour change, but I
don’t consider it to break API, since comparison behaviour isn’t
documented anywhere and nobody’s ever asked.

This includes a test case.
gd: Fix comparison of GDataGDReminders with non-equal relative times

Two GDataGDReminders would previously compare as equal if their methods were
equal but their relative times were not. Now they only compare equal if both
their methods and their relative (or absolute) times are equal.

This includes a test case.

Spotted by Miao Yu <will.yu@yahoo.com>.
tests: Add tests for comparison of GDataGD* objects

Add a number of tests for comparison of GDataGD* objects. This doesn’t
push test coverage up significantly, but allowed validation of the sanity of
the different comparisons when writing the tests.