test: use realpath for NODE_TEST_DIR in common.js

If you don't specify NODE_TEST_DIR, common.tmpDir will resolve to the
realpath of `node/test`.

If you do specify NODE_TEST_DIR (with the environment variable or by
running or by running tools/test.py --test-dir=x), common.tmpDir (which
is resolved from testRoot) uses the symbolic path (doesn't resolve
symlinks). This uses fs.realpathSync() to fix that.

PR-URL: nodejs#10723
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/README.md

@@ -373,7 +373,7 @@ Synchronous version of `spawnPwd`.
 ### tmpDir
 * return [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)
 
-Path to the 'tmp' directory.
+The realpath of the 'tmp' directory.
 
 ### tmpDirName
 * return [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)

test/common.js

@@ -11,7 +11,7 @@ const Timer = process.binding('timer_wrap').Timer;
 const execSync = require('child_process').execSync;
 
 const testRoot = process.env.NODE_TEST_DIR ?
-                   path.resolve(process.env.NODE_TEST_DIR) : __dirname;
+                   fs.realpathSync(process.env.NODE_TEST_DIR) : __dirname;
 
 exports.fixturesDir = path.join(__dirname, 'fixtures');
 exports.tmpDirName = 'tmp';