Add a vala test for generated sources in subdirs

Modified from a test case provided by @benwaffle on
https://github.com/mesonbuild/meson/issues/815

test cases/vala/8 generated source/installed_files.txt

@@ -0,0 +1 @@
+usr/bin/generatedtest

test cases/vala/8 generated source/meson.build

@@ -0,0 +1,7 @@
+project('mytest', 'vala', 'c')
+
+cd = configuration_data()
+cd.set('x', 'y')
+
+subdir('src')
+subdir('tools')

test cases/vala/8 generated source/src/config.vala.in

@@ -0,0 +1,3 @@
+namespace Config {
+    public static const string x = "@x@";
+}

test cases/vala/8 generated source/src/meson.build

@@ -0,0 +1,5 @@
+config = configure_file(input: 'config.vala.in',
+  output: 'config.vala',
+  configuration: cd)
+
+src = files('test.vala')

test cases/vala/8 generated source/src/test.vala

@@ -0,0 +1,3 @@
+void main() {
+    print (Config.x);
+}

test cases/vala/8 generated source/tools/meson.build

@@ -0,0 +1,3 @@
+executable('generatedtest', [src, config],
+  install : true,
+  dependencies: [dependency('glib-2.0'), dependency('gobject-2.0')])