cli: run tests in multiple processes

[yf-castel]

For projects with many independent test cases, running tests takes painfully long. Since test cases are independent, we can run all of them in parallel against one single instance of test validator. This PR implements:

1. a new cmd line option, mp_test, to run tests in parallel.
2. a new option in Anchor.toml, mptest, which specifies the details of multi-process test.

mptest.cmd: the command used to run the tests mptest.tests: path to the folder that contains all test cases. Each test case is a single .ts file.

We launch a single instance of test validator, and launch one process for each .ts file.

[vercel[bot]]

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[charlieyou]

Linking this to the relevant GH issue: https://github.com/coral-xyz/anchor/issues/637

[Henry-E]

At minimum this needs a new tests or to be applied to one of the existing tests with multiple test suites. Plus a changelog entry.