[Fix] npm test on windows

The version of "nyc" (v9) that was being used was not launching `tap` properly on windows. So a dev dependency has been added for the latest nyc (v15), which appears to fix the issue.

Note that version 9 of "nyc" was actually being installed indirectly, as a dependency of "tap". I think using indirect dependencies is a bad idea, and also it appears that newer versions of tap no longer have nyc as a dependency anyway. (Updating tap is a lot more work, so I want to avoid that for now).

Also, the windows shell does not understand single quotes, so an extra npm run script is required.

package.json

@@ -64,6 +64,7 @@
 		"js-yaml": "^3.14.0",
 		"npm-run-posix-or-windows": "^2.0.2",
 		"npmignore": "^0.3.0",
+		"nyc": "^15.1.0",
 		"safe-publish-latest": "^2.0.0",
 		"tap": "^8.0.1",
 		"tap-parser": "^5.4.0"
@@ -80,9 +81,10 @@
 		"prelint": "npm-run-posix-or-windows eclint",
 		"lint": "eslint --ext .js,.cjs,.mjs . bin/*",
 		"pretest": "npm run lint",
-		"test": "npm run tests-only",
+		"test": "npm-run-posix-or-windows tests-only",
 		"posttest": "aud --production",
 		"tests-only": "nyc tap 'test/*.js'",
+		"tests-only:windows": "nyc tap test/*.js",
 		"test:example": "find example -name '*.js' | grep -v fail | grep -v static | xargs tap"
 	},
 	"testling": {