update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/simple/register_target_in_build_hook.rb

@@ -0,0 +1,8 @@
+Rscons::Environment.new do |env|
+  env.Program("simple.exe", Dir["*.c"])
+  env.add_build_hook do |build_op|
+    if build_op[:target].end_with?(".o")
+      env.Disassemble("#{build_op[:target]}.txt", build_op[:target])
+    end
+  end
+end