fix: run each test file in its own process

A test file's module state leaked into the next one because they were
all loaded into the same process. With `@std/testing/bdd` this meant
the global hooks of the second file either errored with "Cannot add
global hooks after a global test is registered" or silently registered
tests that never ran.

`deno test` gives each test file its own isolate, so do the equivalent
by spawning a process per file.

Closes #432

Author: dsherret

deno.jsonc

@@ -58,6 +58,7 @@
     "tests/polyfill_disabled_project/npm",
     "tests/polyfill_project/npm",
     "tests/shim_project/npm",
+    "tests/test_hooks_project/npm",
     "tests/test_project/npm",
     "tests/tla_project/npm",
     "tests/types_package_project/npm",