feat: add a testIsolation build option to run each test file in its own process

[dsherret]

Closes #432

The test runner loads every test file into the same process, so a test file's module state leaks into the next one. With @std/testing/bdd the second file's global hooks hit TestSuiteInternal.started:

Error: Cannot add global hooks after a global test is registered

...or, when the first file already created the global suite, the hooks silently attach to the already drained global test so those tests never run:

Running tests in ./script/b.test.js...

Running tests in ./esm/b.test.js...

deno test gives each test file its own isolate. Since doing the equivalent means starting a process per test file, it's opt in:

await build({
  // ...etc...
  testIsolation: "process",
});

The generated runner then re-spawns itself once per test file. The children inherit stdio so the output is the same, the node arguments of the parent are forwarded, and the parent exits non-zero when any file fails. A preload module is loaded before each test file in this mode, which is what the isolation implies.

The default ("none") keeps the current behavior of running everything in one process, so nothing changes for anyone who doesn't opt in.

Added tests/test_hooks_project, where the second test file adds a global hook. Loading both of its output files in one process errors with the message above, and the fixture passes with testIsolation: "process".