Add feature gating for slow tests and cache SRS using git LFS

[willemolding]

Test runs were previously using too much time/memory and getting killed by GitHub.

This PR:

• Adds new cargo features: 'test-full-prover' and 'test-evm-verifier' which must be enabled to test using the full prover and prover+evm verifier respectively. These will not be enabled by default or for CI tests
• Adds the .srs files to the repo using git LFS. Having these cached speeds up the tests significantly compared with regenerating them each time. LFS ensures this does not bloat the git history but also allows the files to be fetched when needed.

[willemolding]

Possibly this is still being killed for using too much memory.. Error: Process completed with exit code 143.

[nulltea]

Thanks for this PR, @willemolding! Let me check if changing the circuit degree will help with memory limit

[nulltea]

CI tests now run mock prover with no SRS or prover keys loaded, thus these changes are no longer needed. Thanks anyway :)