execute all build tests in the top-level rake context instead of spawning subprocesses

build_tests/two_sources/build.rb

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