tests: add Vala thread flag test

This test checks that things don’t fall apart when a threads dependency
is added to a Vala target.

test cases/vala/23 thread flags/meson.build

@@ -0,0 +1,7 @@
+project('thread flags', 'c', 'vala')
+
+glib = dependency('glib-2.0')
+gobject = dependency('gobject-2.0')
+threads = dependency('threads')
+
+e = executable('valaprog', 'prog.vala', dependencies: [glib, gobject, threads])

test cases/vala/23 thread flags/prog.vala

@@ -0,0 +1,2 @@
+void main () {
+}