test_runner: support running tests in process
Opening as a draft. Some of this logic is from https://github.com/nodejs/node/pull/53921. I also still need to finish docs and tests. I've added @MoLow as a co-author because the work in https://github.com/nodejs/node/pull/51579 was very helpful.
This commit introduces a new --experimental-test-isolation flag that, when set to 'none', causes the test runner to execute all tests in the same process. By default, this is the main test runner process, but if watch mode is enabled, it spawns a separate process that runs all of the tests.
The default value of the new flag is 'process', which uses the existing behavior of running each test file in its own child process.
It is worth noting that when the isolation mode is 'none', globals and all other top level logic (such as top level hooks) is shared among all files.