Improve PR build pipeline

[jordy25519]

Pipeline often fails with timeouts ~2 hours on the test step.

For comparison a clean local build takes around 35mins and a test ~40mins (requires building and executing tests)

Running the test suite in 1 hour seems like a reasonable target

Some options to improve this:

• Upgrade circleci plan with more CPU allocation
• Switch CI altogether to get more CPU (parity uses gitlab)
• Optimize caching with sccache
• Chunk tests by crates. we run them all in one step cargo test --all at the moment

[jordy25519]

Relevant circleci docs about requesting CPU allocation: https://circleci.com/docs/2.0/configuration-reference/#executors-requires-version-21