Add testsuite skeleton

https://bugzilla.gnome.org/show_bug.cgi?id=730447
wall-clock: Always use the ratio character for time strings

Since the ratio character looks visually more pleasing in time strings,
replace colons with the ratio characters whenever possible.

https://bugzilla.gnome.org/show_bug.cgi?id=726232
tests: Add a wall clock test for colon vs ratio character use

https://bugzilla.gnome.org/show_bug.cgi?id=730447
tests: Add a wall clock test for 12/24h gsettings switch

https://bugzilla.gnome.org/show_bug.cgi?id=730447
tests: Add a wall clock test for the notify::clock callback

https://bugzilla.gnome.org/show_bug.cgi?id=730447
test-wall-clock: Fix compile-time warning

Innocuous, but annoying.
wall-clock: Export gnome_wall_clock_string_for_datetime()

gnome_wall_clock_string_for_datetime() will be used for debugging
purposes.
tests: Add gtk-reftest to compare GTK+ outputs

For each UI file, we'll compare the GTK+ output from the wall-clock for
that particular locale with a reference UI file.

This way, we can test whether we regress in terms of RTL handling.
tests: Add reftest UI files

We'll test a few locales and their outputs.