test: decrease inconsistency in the common.js

1. `process.env['PROCESSOR_ARCHITEW6432']` ->
   `process.env.PROCESSOR_ARCHITEW6432`.

2. `path.dirname(__filename)` -> `__dirname`.

3. `path.resolve(__dirname)` -> `__dirname`.

PR-URL: #7758
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/common.js

@@ -9,17 +9,17 @@ const stream = require('stream');
 const util = require('util');
 const Timer = process.binding('timer_wrap').Timer;
 
-const testRoot = path.resolve(process.env.NODE_TEST_DIR ||
-                              path.dirname(__filename));
+const testRoot = process.env.NODE_TEST_DIR ?
+                   path.resolve(process.env.NODE_TEST_DIR) : __dirname;
 
-exports.testDir = path.dirname(__filename);
+exports.testDir = __dirname;
 exports.fixturesDir = path.join(exports.testDir, 'fixtures');
 exports.libDir = path.join(exports.testDir, '../lib');
 exports.tmpDirName = 'tmp';
 exports.PORT = +process.env.NODE_COMMON_PORT || 12346;
 exports.isWindows = process.platform === 'win32';
 exports.isWOW64 = exports.isWindows &&
-                  (process.env['PROCESSOR_ARCHITEW6432'] !== undefined);
+                  (process.env.PROCESSOR_ARCHITEW6432 !== undefined);
 exports.isAix = process.platform === 'aix';
 exports.isLinuxPPCBE = (process.platform === 'linux') &&
                        (process.arch === 'ppc64') &&