Add a spec for escaped colons in a C constant comment

`YARD::Handlers::C::HandlerMethods#find_constant_docstring` has a beefy regex that allows a comment to consist of a value and a description, separated by a colon. The regex also allows the user to escape colons as well, in case the value has a colon in it, as described in a comment:

```ruby
# In the case of rb_define_const, the definition and comment are in
# "/* definition: comment */" form.  The literal ':' and '\' characters
# can be escaped with a backslash.
```

This tests said behavior.