Ignore if `conf.enable_test` does not exist

Because if the configuration file didn't contain any `conf.enable_test`, `rake test` would report an exception and exit.

```console
% cat my_config.rb
MRuby::Build.new { toolchain }

% rake MRUBY_CONFIG=my_config.rb test

  ...SNIP...

rake aborted!
NoMethodError: undefined method `invoke' for nil:NilClass
/var/tmp/mruby/tasks/test.rake:24:in `block (3 levels) in <top (required)>'
Tasks: TOP => test => test:build => test:build:lib
(See full trace by running task with --trace)
```

0 files changed, 0 insertions, 0 deletions