refactor: dry up test suite, keep same function
refactor: dry up test suite, keep same function

This PR seeks to DRY up `srcset_test.rb`. The general idea is to DRY
the test suites up  for:

* pattern ergonomics
* correctness
* (possibly) introducing [RSpec](https://rspec.info)

**Commonly used functionality and constants** have been pulled up into
the `SrcsetTest` module.

* `mock_client` appears 17 times
* `mock_signed_client` appears 21 times
* `get_expected_signature` appears 6 times
  * this function composes the previous method into 3 functions:
    * `get_sig_from(src)`
    * `get_params_from(src)`
    * and finally returns the result of the MD5 operation
* `RESOLUTIONS` and `DPR_QUALITY` are now module-level, frozen,
  constants

This PR also complies with many RuboCop suggestions (but not all).

The suggestions I elected to commit are mostly to:
* improve readability
  * corral line lengths
  * use single-quoted strings (where interpolation is not needed)
  * align chained calls (which there are fewer of now)
  * use consistent indentation
* seek idiomatic code

imgix/imgix-rb