test: replace fixturesDir with fixtures module

PR-URL: https://github.com/nodejs/node/pull/15961
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-npm-install.js

@@ -7,6 +7,7 @@ const path = require('path');
 const exec = require('child_process').exec;
 const assert = require('assert');
 const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 common.refreshTmpDir();
 const npmSandbox = path.join(common.tmpDir, 'npm-sandbox');
@@ -26,7 +27,7 @@ const npmPath = path.join(
 
 const pkgContent = JSON.stringify({
   dependencies: {
-    'package-name': `${common.fixturesDir}/packages/main`
+    'package-name': fixtures.path('packages/main')
   }
 });