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

build_tests/library/build.rb

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