update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/custom_builder/Rsconsfile

@@ -0,0 +1,16 @@
+class MySource < Rscons::Builder
+  def run(target, sources, cache, env, vars)
+    File.open(target, 'w') do |fh|
+      fh.puts <<EOF
+#define THE_VALUE 5678
+EOF
+    end
+    target
+  end
+end
+
+Rscons::Environment.new do |env|
+  env.add_builder(MySource.new)
+  env.MySource('inc.h', [])
+  env.Program('program.exe', Dir['*.c'])
+end