gtestutils: make the new assert messages more detailed

g_assert_true(), g_assert_false(), g_assert_null(), and
g_assert_nonnull() simply printed out the expression they were
checking, without any further explanation of what went wrong. (In
particular, "g_assert_true(x)" and "g_assert_false(x)" would both
print the same thing on failure.) Add a little bit more context.

https://bugzilla.gnome.org/show_bug.cgi?id=724385