Add pre-generated RSA keys for testing

We have a variety of tests in the RSA tests that do key generation to obtain a specific key size that is then used for signing/verification/encryption/decryption. We really shouldn't be generating in these cases for the following reasons:

* Alternate backends may not be able to generate the needed key sizes (but can load existing keys of that size without issues. This is important for the upcoming CommonCrypto expansion)
* It mixes multiple concepts in a single test (generation failures should not affect signing tests, et cetera)
* It's slower

This PR adds pre-generated RSA keys and places them in a `fixtures_rsa` file.

Refs #1074, #1078