simplify core detection

avoid detecting the os and instead just use:

```ruby
from_env || from_sysdev || from_sysctl
```

Notes:
1. Since `$?` can't be stubbed, the workaround is to use \`true\` and \`false\` in the shell.
2. I couldn't get `it {should raise_error}` shorthand working , thus the `specify`