update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/library/Rsconsfile

@@ -0,0 +1,4 @@
+Rscons::Environment.new(echo: :command) do |env|
+  env.Program('library.exe', ['lib.a', 'three.c'])
+  env.Library("lib.a", ['one.c', 'two.c'], 'CPPFLAGS' => ['-Dmake_lib'])
+end