test: Add basic SYNC tests.

I couldn't find any, and I was modifying the implementation, so I had
to write some.  I would like the test to end with a "make sure there
weren't any stray unchecked errors", but I didn't figure out how to do
that.

v2: Extend sync tests to cover alarm delta and waitvalue changes.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

test/meson.build

@@ -22,3 +22,5 @@ if get_option('xvfb')
         )
     endif
 endif
+
+subdir('sync')