Run Tests Sequentially

[adrian-burlacu-software]

What is the problem this feature will solve?

Some singletons cannot be tested in parallel through the node:test runner:

• admin features
• files for test data
• DBs like SQLite that cannot write concurrently and throw errors

The current test runner runs all the tests in parallel.

What is the feature you are proposing to solve the problem?

Add a --serial flag to maintain single-threaded nature of JS in the test runner by running a single test file(in a child process) at a time not all at once as it does currently.

What alternatives have you considered?

Running tests in the same process, such as the discussion in issue #48871.

[cjihrig]

Closing as a duplicate of #49487.