Add a test that contains only generated Vala files

Demonstrates that the Vala detection is broken when all the files are
generated.

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

@@ -1 +1,2 @@
 usr/bin/generatedtest
+usr/bin/onlygentest

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

@@ -5,3 +5,4 @@ cd.set('x', 'y')
 
 subdir('src')
 subdir('tools')
+subdir('onlygen')

test cases/vala/8 generated sources/onlygen/maingen.in

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

test cases/vala/8 generated sources/onlygen/meson.build

@@ -0,0 +1,7 @@
+onlygen = generator(copy,
+  output : '@BASENAME@.vala',
+  arguments : ['@INPUT@', '@OUTPUT@'])
+
+executable('onlygentest', onlygen.process('maingen.in'),
+  install : true,
+  dependencies: [dependency('glib-2.0'), dependency('gobject-2.0')])