test: add common.rootDir

A few of the child process tests can be simplified by computing
the OS specific root directory in common and then accessing that
value.

PR-URL: https://github.com/nodejs/node/pull/7685
Reviewed-By: Roman Reiss <me@silverwind.io>

test/common.js

@@ -27,6 +27,7 @@ exports.isSunOS = process.platform === 'sunos';
 exports.isFreeBSD = process.platform === 'freebsd';
 
 exports.enoughTestMem = os.totalmem() > 0x20000000; /* 512MB */
+exports.rootDir = exports.isWindows ? 'c:\\' : '/';
 
 function rimrafSync(p) {
   try {

test/parallel/test-child-process-cwd.js

@@ -36,12 +36,11 @@ function testCwd(options, forCode, forData) {
 }
 
 // Assume these exist, and 'pwd' gives us the right directory back
+testCwd({cwd: common.rootDir}, 0, common.rootDir);
 if (common.isWindows) {
   testCwd({cwd: process.env.windir}, 0, process.env.windir);
-  testCwd({cwd: 'c:\\'}, 0, 'c:\\');
 } else {
   testCwd({cwd: '/dev'}, 0, '/dev');
-  testCwd({cwd: '/'}, 0, '/');
 }
 
 // Assume does-not-exist doesn't exist, expect exitCode=-1 and errno=ENOENT

test/parallel/test-child-process-spawnsync.js

@@ -20,7 +20,7 @@ assert.deepStrictEqual(ret_err.spawnargs, ['bar']);
 
 {
   // Test the cwd option
-  const cwd = common.isWindows ? 'c:\\' : '/';
+  const cwd = common.rootDir;
   const response = common.spawnSyncPwd({cwd});
 
   assert.strictEqual(response.stdout.toString().trim(), cwd);

test/sequential/test-child-process-execsync.js

@@ -61,20 +61,13 @@ assert.strictEqual(ret, msg + '\n',
                    'execFileSync encoding result should match');
 
 // Verify that the cwd option works - GH #7824
-(function() {
-  var response;
-  var cwd;
-
-  if (common.isWindows) {
-    cwd = 'c:\\';
-    response = execSync('echo %cd%', {cwd: cwd});
-  } else {
-    cwd = '/';
-    response = execSync('pwd', {cwd: cwd});
-  }
+{
+  const cwd = common.rootDir;
+  const cmd = common.isWindows ? 'echo %cd%' : 'pwd';
+  const response = execSync(cmd, {cwd});
 
   assert.strictEqual(response.toString().trim(), cwd);
-})();
+}
 
 // Verify that stderr is not accessed when stdio = 'ignore' - GH #7966
 (function() {