Add messages to `unsupported_before` and `supported_before` helpers (#1225)
Add messages to `unsupported_before` and `supported_before` helpers (#1225)

The existing message doesn't give any context as to why it's skipped, so this adds some explanation.

**Before:**

```
Pending: (Failures listed here are expected and do not affect your suite's status)

  1) Magick::Image#channel_entropy raises an error on earlier versions
     # No reason given
     # ./spec/rmagick/image/channel_entropy_spec.rb:36
```

**After:**

```
Pending: (Failures listed here are expected and do not affect your suite's status)

  1) Magick::Image#channel_entropy raises an error on earlier versions
     # Supported before 6.9.0; running 7.0.10
     # ./spec/rmagick/image/channel_entropy_spec.rb:36
```
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick