Add yarn flag to the anchor test command

[michaelhly]

Add option to run anchor test via yarn.

$ ../../anchor/cli/target/debug/anchor test --help
anchor-test 
Runs integration tests against a localnetwork

USAGE:
    anchor test [FLAGS] [file]

ARGS:
    <file>    

FLAGS:
    -h, --help                    Prints help information
        --skip-deploy             Use this flag if you want to run tests against previously deployed
                                  programs
        --skip-local-validator    Flag to skip starting a local validator, if the configured cluster
                                  url is a localnet
    -V, --version                 Prints version information
        --yarn                    Use this flag if you want to use yarn as your JavaScript package manager

Test:

$ ../../anchor/cli/target/debug/anchor test --yarn
BPF SDK: /Users/user/.local/share/solana/install/releases/1.6.6/solana-release/bin/sdk/bpf
Running: rustup toolchain list -v
Running: cargo +bpf build --target bpfel-unknown-unknown --release
    Finished release [optimized] target(s) in 0.76s

To deploy this program:
  $ solana program deploy /Users/user/Projects/monorepo/lockup/target/deploy/lockup.so
yarn run v1.22.10
$ /Users/user/Projects/monorepo/lockup/node_modules/.bin/mocha -t 1000000 tests/

  Lockup and Registry
    ✓ Sets up initial test state (353ms)
    ✓ Is initialized! (503ms)
    ✓ Deletes the default whitelisted addresses (495ms)
    ✓ Sets a new authority (1010ms)

...