The cause is that `mrbgem.rake` of` mruby-test` gem is loaded when test
code is requested to be built, but when `mrbgem.rake` is loaded,
`MRuby::Gem.current` is updated, which is not thread safe.
Address this by not loading `mrbgem.rake` in parallel.
With this change, the test code will not be built unless `rake test` is
run, so there will be almost no side effects even if `enable_test` is
always set (but, gems specified by `add_test_dependency` are included
in `libmruby.a`).
Also added are `test: build` task, which only builds the test code
(including the main code), and `test: run` task, which only runs tests
independent of build. Therefore, the idiom for building in parallel and
not running tests in parallel is `rake -m test:build && rake test:run`.
KOBAYASHI Shuji