test: switch to use common.fixtures.fixturesDir

code and learn 2017 first task is to switch from common.fixturesDir to
common.fixtures.fixturesDir in test-https-strict.js

PR-URL: #15814
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-https-strict.js

@@ -1,5 +1,6 @@
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
@@ -8,15 +9,9 @@ process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0';
 
 const assert = require('assert');
 const https = require('https');
-const fs = require('fs');
-const path = require('path');
-
-function file(fname) {
-  return path.resolve(common.fixturesDir, 'keys', fname);
-}
 
 function read(fname) {
-  return fs.readFileSync(file(fname));
+  return fixtures.readKey(fname);
 }
 
 // key1 is signed by ca1.