test: replace fixturesDir with fixtures module

PR-URL: #15919
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/sequential/test-debugger-debug-brk.js

@@ -1,10 +1,13 @@
 'use strict';
 const common = require('../common');
 common.skipIfInspectorDisabled();
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-const script = `${common.fixturesDir}/empty.js`;
+// file name here doesn't actually matter since
+// debugger will connect regardless of file name arg
+const script = fixtures.path('empty.js');
 
 function test(arg) {
   const child = spawn(process.execPath, ['--inspect', arg, script]);