update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/two_sources/Rsconsfile

@@ -0,0 +1,4 @@
+Rscons::Environment.new(echo: :command) do |env|
+  env.Object("one.o", "one.c", 'CPPFLAGS' => ['-DONE'])
+  env.Program('two_sources.exe', ['one.o', 'two.c'])
+end