Add a Vala test case for mixed sources

test cases/vala/11 mixed sources/bar.vala

@@ -0,0 +1,5 @@
+extern int test ();
+
+public int main (string[] args) {
+	return test ();
+}

test cases/vala/11 mixed sources/foo.c

@@ -0,0 +1,3 @@
+int test () {
+    return 0;
+}

test cases/vala/11 mixed sources/meson.build

@@ -0,0 +1,5 @@
+project('foo', 'c', 'vala')
+
+glib = dependency('glib-2.0')
+
+executable('foo', 'foo.c', 'bar.vala', dependencies: [glib])