Fix libdeflate feature selection

[juliangehring]

The libdeflate feature (added in #189) isn't currently propagated and the feature gets silently ignored during build. Fixing the feature names and dependencies addresses this, and also ensures that the feature-specific tests are correctly ignored.

If someone knows a good way to automatically test that the correct deflate library is included, I'd be happy to add this as well.

[coveralls]

Pull Request Test Coverage Report for Build 143906896

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.591%

---

Totals
Change from base Build 143771216:	0.0%
Covered Lines:	10172
Relevant Lines:	10986

---

💛 - Coveralls

[brainstorm]

Good catch @juliangehring, thanks!

If someone knows a good way to automatically test that the correct deflate library is included, I'd be happy to add this as well.

That should be tackled by libdeflate-sys itself afaik (it'll error out if not found or sth else is wrong)? Regardless, let's merge this PR, looks fine and seems to work now (local test), thanks again!

[brainstorm]

Oh, sorry, you meant testing as in use libflate and assert some result on tests?