Fixes #3859.

This isn't just about adding support for parallel testing. When running all tests simultaneously, many tests proved to be fragile and dependent on external tests being run beforehand (by other files).

By forcing a random execution order (even if still sequentially), several tests began to fail due to tests that affect MySQL Server global variables/states. These tests have been isolated in ./test/esm/global/ (temporary path until the .cjs files are completely removed).

• Now they perform the necessary global changes in isolation and restore them to their original state at the end of testing, ensuring that no compromised tests can compromise or cause false positives in unrelated tests.

Now the tests run in parallel and, naturally, tend to finish faster since they are launched simultaneously.

Together, I fixed an incorrect structure in the creation and closing of connections, mentioned in #3859. Basically, the challenge was to ensure that no connection is created or closed in the same scope that may contain intentional failures (e.g., assertions) in the tests, for example:

❌ Current approach

import { describe, it, assert } from 'poku';

await describe('test', async () => {
  it('should do something', () => {
    const connection = await createConnection();

    assert(false); // will fail here

    await connection.end(); // the connection will never be reached
  });

  // the process will remain hanging indefinitely
});

✨ New approach

import { describe, it, assert } from 'poku';

await describe('test', async () => {
  const connection = await createConnection();

  it('should do something', () => {
    assert(false); // will fail in its own scope
  });

  await connection.end();
});

• The same idea applies to callbacks, but with more complex approaches adapted to each test.

Special thanks to @HappyZombies, who identified the issue and now it will be possible to enable all tests for Bun (not for this PR) 🎉