fix: close the child process

[jackfiszr]

Not closing the process causes:

error: AssertionError: Test case is leaking 3 resources:

 - A child process stdout (rid 3) was opened during the test, but not closed during the test. Close the child process stdout by calling `proc.stdout.close()`.      
 - A child process stderr (rid 4) was opened during the test, but not closed during the test. Close the child process stderr by calling `proc.stderr.close()`.      
 - A child process (rid 5) was started during the test, but not closed during the test. Close the child process by calling `proc.kill()` or `proc.close()`.

    at assert (deno:ext/web/00_infra.js:294:13)
    at resourceSanitizer (deno:runtime/js/40_testing.js:407:7)
    at async Object.exitSanitizer [as fn] (deno:runtime/js/40_testing.js:425:9)
    at async runTest (deno:runtime/js/40_testing.js:831:7)
    at async Object.runTests (deno:runtime/js/40_testing.js:1084:22)

...when testing code that uses the library.