fix coverage for backend parametrization

Any test that takes a backend is required to provide the
requires_backend_interface decorator.
Merge pull request #1446 from reaperhulk/fix-coverage

fix coverage for backend parametrization
Added attributes that are part of hte interface to several test fakes
Undo change that's unrelatedo this branch
Merge pull request #1448 from alex/fakes-need-attributes-too

Added attributes that are part of hte interface to several test fakes
Merge branch 'master' into verify-interfaces

Conflicts:
	tests/hazmat/backends/test_commoncrypto.py
	tests/hazmat/backends/test_openssl.py
	tests/hazmat/primitives/test_block.py
	tests/hazmat/primitives/test_hashes.py
	tests/hazmat/primitives/test_hmac.py
	tests/hazmat/primitives/test_pbkdf2hmac.py
many verify, for now