Our CI already run test files in parallel, having `node:test` spawns
child processes concurrently could lead to oversubscribing the CI
machine. This commit sets the `concurrency` depending
on the presence of `TEST_PARALLEL` in the env, so running the test
file individually still spawns child processes concurrently, and
running the whole test suite does not oversubscribe the machine.
PR-URL: https://github.com/nodejs/node/pull/52177
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
