test: fixtures in test-process-redirect-warnings-env

Replaced common.fixturesDir with common.fixtures in
test-process-redirect-warnings-env

PR-URL: #15930
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-process-redirect-warnings-env.js

@@ -6,14 +6,15 @@
 // opened and the contents are validated
 
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 const fs = require('fs');
 const fork = require('child_process').fork;
 const path = require('path');
 const assert = require('assert');
 
 common.refreshTmpDir();
 
-const warnmod = require.resolve(`${common.fixturesDir}/warnings.js`);
+const warnmod = require.resolve(fixtures.path('warnings.js'));
 const warnpath = path.join(common.tmpDir, 'warnings.txt');
 
 fork(warnmod, { env: Object.assign({}, process.env,