lib: do a full warmup run to reduce variance on first benchmark run

[RafaelGSS]

Fixes: #12

[RafaelGSS]

Before this fix:

test at test/env.js:12:3
✖ must have a similar benchmark result (0.88ms)
  AssertionError [ERR_ASSERTION]: 213241088.30334634 too different from 129032871.41930455 - Using includes
      at TestContext.<anonymous> (/Users/rafaelgss/repos/os/bench-node/test/env.js:24:18)
      at Test.runInAsyncScope (node:async_hooks:211:14)
      at Test.run (node:internal/test_runner/test:930:25)
      at async Promise.all (index 0)
      at async Suite.run (node:internal/test_runner/test:1295:7)
      at async startSubtestAfterBootstrap (node:internal/test_runner/harness:289:3) {
    generatedMessage: false,
    code: 'ERR_ASSERTION',
    actual: false,
    expected: true,
    operator: '=='
  }