date test: Use g_test_skip(), not g_test_incomplete()
g_test_skip() is appropriate for tests that can't run due to missing functionality on the host system, whereas g_test_incomplete() is for expected-to-fail tests for unimplemented functionality in the code under test.
See also https://bugzilla.gnome.org/show_bug.cgi?id=793635 and commit 5459b345.
Signed-off-by: Simon McVittie smcv@collabora.com