test: add script to run tests and leave fixtures

This is particularly handy when tests fail, so that we can inspect the
fixtures that they were using.

package.json

@@ -292,7 +292,9 @@
     "tap-cover": "tap -J --nyc-arg=--cache --coverage --timeout 600 --no-esm",
     "pretest": "standard",
     "test": "npm run test-tap --",
+    "test:nocleanup": "NO_TEST_CLEANUP=1 npm run test --",
     "sudotest": "sudo npm run tap -- \"test/tap/*.js\"",
+    "sudotest:nocleanup": "sudo NO_TEST_CLEANUP=1 npm run tap -- \"test/tap/*.js\"",
     "posttest": "rimraf test/npm_cache*",
     "test-coverage": "npm run tap-cover -- \"test/tap/*.js\" \"test/network/*.js\" \"test/broken-under-*/*.js\"",
     "test-tap": "npm run tap -- \"test/tap/*.js\" \"test/network/*.js\" \"test/broken-under-*/*.js\"",