test: make sure free ports get used

[fry69]

This fixes issues I have with running tests on one of my servers, where port 8000 on localhost is occupied.

Note: The current init test will continue to fail, but that will hopefully get replaced by #3524 which does not have this problem.

Before:

[
  "error: Uncaught (in promise) AddrInUse: Address already in use (os error 98)",
  "    at listen (ext:deno_net/01_net.js:594:35)",
  "    at serveInner (ext:deno_http/00_serve.ts:675:16)",
  "    at Object.serve (ext:deno_http/00_serve.ts:591:10)",
  "    at ext:deno_http/00_serve.ts:819:10",
  "    at ext:runtime_main/js/99_main.js:708:13",
  "    at eventLoopTick (ext:core/01_core.js:179:7)"
]
[...]
Builder - write prod routePattern => ./packages/fresh/src/dev/builder_test.ts:284:6
Builder - file:// islands => ./packages/fresh/src/dev/builder_test.ts:319:6
Builder - serves static files in subdir in prod => ./packages/fresh/src/dev/builder_test.ts:474:6
init - can start built project => ./packages/init/src/init_test.ts:237:6

FAILED | 471 passed (14 steps) | 4 failed | 18 ignored

After:

ok | 475 passed (14 steps) | 0 failed | 18 ignored

[marvinhagemeister]

LGTM