update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/clone_env/Rsconsfile

@@ -0,0 +1,12 @@
+debug = Rscons::Environment.new(echo: :command) do |env|
+  env.build_dir('src', 'debug')
+  env['CFLAGS'] = '-O2'
+  env['CPPFLAGS'] = '-DSTRING="Debug Version"'
+  env.Program('program-debug.exe', Dir['src/*.c'])
+end
+
+release = debug.clone do |env|
+  env["CPPFLAGS"] = '-DSTRING="Release Version"'
+  env.build_dir('src', 'release')
+  env.Program('program-release.exe', Dir['src/*.c'])
+end