update run_builder, build_sources integration tests for more coverage

build_tests/simple/build_sources.rb

@@ -10,5 +10,13 @@ end
 
 Rscons::Environment.new do |env|
   env.add_builder(MyProgram.new)
-  env.MyProgram("simple.exe", "simple.c")
+  env.Object("simple.o", "simple.c")
+  File.open("two.c", "wb") do |fh|
+    fh.puts <<-EOF
+      void two(void)
+      {
+      }
+    EOF
+  end
+  env.MyProgram("simple.exe", ["simple.o", "two.c"])
 end