tests: Enable tests for videodecoder

The tests pass fine here so don't see any reason to keep them disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=795005
diff --git a/tests/check/meson.build b/tests/check/meson.build
index de8f66a..fb9c153 100644
--- a/tests/check/meson.build
+++ b/tests/check/meson.build
@@ -25,6 +25,7 @@
   [ 'libs/tag.c' ],
   [ 'libs/video.c' ],
   [ 'libs/videoencoder.c' ],
+  [ 'libs/videodecoder.c' ],
   [ 'libs/videotimecode.c' ],
   [ 'libs/xmpwriter.c' ],
   [ 'elements/adder.c' ],