gtkicontheme: Don’t emit warning about fallback theme when in unit tests

gtkicontheme: Don’t emit warning about fallback theme when in unit tests

In order to eliminate g_test_expect_message() (which doesn’t work with
G_LOG_USE_STRUCTURED), make the warning about the fallback theme not
existing be conditional on the icon theme search path containing a
system path. Any application code which modifies the search path does so
through appends and prepends, so this should not affect whether the
warning is emitted in production.

https://bugzilla.gnome.org/show_bug.cgi?id=769485