update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/build_dir/Rsconsfile

@@ -0,0 +1,5 @@
+Rscons::Environment.new do |env|
+  env.append('CPPPATH' => Dir['src/**/*/'].sort)
+  env.build_dir(%r{^src/([^/]+)/}, 'build_\\1/')
+  env.Program('build_dir.exe', Dir['src/**/*.c'])
+end