Test cases for propertyNames with pattern.

[LeifRilbe]

Added test cases for propertyNames with pattern.
Ref networknt/json-schema-validator#326

[notEthan]

I think this is a good and valuable test to add. pattern is probably the most common keyword to be used
in a propertyNames schema, I would guess, so good to have in here.

I'd remove the tests for 'ignores string/array/etc' - I think these are covered by the tests both of the pattern keyword and the test above of propertyNames + maxLength.

one passinng and one failing test seems sufficient - the multiple fail cases feel more like testing the regex itself, rather than the application of the keyword. to that point, a less complex regex than the camelCase one might be simpler and more readable, such as the one used by the pattern tests.

note that this comment is not a change request. I'm not active enough in this repository to be telling anybody else what to write. just a couple of my thoughts.

[Julian]

Can you remove the reference to camel case in the test names since it seems we managed to simplify things?

After that yeah this looks great!