vala: better test case

Closes Homebrew/homebrew#29339.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/vala.rb

@@ -6,7 +6,6 @@ class Vala < Formula
   url "http://ftp.acc.umu.se/pub/gnome/sources/vala/0.24/vala-0.24.0.tar.xz"
   sha1 "33a71a21e12e80cf1f4e0aa3b6a6523ff38e92c8"
 
-
   depends_on "pkg-config" => :build
   depends_on "gettext"
   depends_on "glib"
@@ -19,6 +18,26 @@ class Vala < Formula
   end
 
   test do
-    system "#{bin}/valac", "--version"
+    test_string = "Hello Homebrew\n"
+    path = testpath/"hello.vala"
+    path.write <<-EOS
+      void main () {
+        print ("#{test_string}");
+      }
+    EOS
+    valac_args = [# Build with debugging symbols.
+                  "-g",
+                  # Use Homebrew's default C compiler.
+                  "--cc=#{ENV.cc}",
+                  # Save generated C source code.
+                  "--save-temps",
+                  # Vala source code path.
+                  "#{path}"]
+    system "#{bin}/valac", *valac_args
+    assert File.exist?(testpath/"hello.c")
+
+    output = `#{testpath}/hello`
+    assert_equal test_string, output
+    assert_equal 0, $?.exitstatus
   end
 end