test: skip fs.watch intensive tests on Travis

We skip 3 tests that make intensive use of FS watcher on Travis because
it has a limit of the number of watchers that can be used.

Refs: travis-ci.com/nodejs/node/jobs/142234998
Refs: docs.travis-ci.com/user/environment-variables/#default-environment-variables

Author: refack

test/async-hooks/test-fseventwrap.js

@@ -10,6 +10,9 @@ const fs = require('fs');
 if (!common.isMainThread)
   common.skip('Worker bootstrapping works differently -> different async IDs');
 
+if (process.env.TRAVIS)
+  common.skip('Skip fs.watch-intensive test because of Travis limitation.');
+
 const hooks = initHooks();
 
 hooks.enable();

test/parallel/test-fs-watch-close-when-destroyed.js

@@ -8,6 +8,9 @@ const tmpdir = require('../common/tmpdir');
 const fs = require('fs');
 const path = require('path');
 
+if (process.env.TRAVIS)
+  common.skip('Skip fs.watch-intensive test because of Travis limitation.');
+
 tmpdir.refresh();
 const root = path.join(tmpdir.path, 'watched-directory');
 fs.mkdirSync(root);

test/sequential/test-fs-watch.js

@@ -31,6 +31,9 @@ const tmpdir = require('../common/tmpdir');
 if (!common.isMainThread)
   common.skip('process.chdir is not available in Workers');
 
+if (process.env.TRAVIS)
+  common.skip('Skip fs.watch-intensive test because of Travis limitation.');
+
 const expectFilePath = common.isWindows ||
                        common.isLinux ||
                        common.isOSX ||