Adding test for external basedir case

test/external_basedir.js

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+var browserify = require('../');
+var vm = require('vm');
+var test = require('tap').test;
+
+test('external', function (t) {
+    t.plan(2);
+    var baseDir = __dirname;
+    var splitFile = baseDir + '/external_basedir/y.js';
+    var mainSrc = '';
+    var splitSrc = '';
+    var pending = 2;
+
+    // main bundle should not include splitFile
+    var b = browserify(baseDir + '/external_basedir/main.js', {basedir: baseDir});
+    b.external(splitFile); // no need to specify basedir option if it is already set for b
+    b.bundle(function (err, src) {
+        if (err) return t.fail(err);
+        mainSrc = src;
+        pending--;
+        done();
+    });
+
+    // consider b2 as for split or vendor bundle
+    var b2 = browserify([], {basedir: baseDir});
+    b2.require(splitFile, {expose: splitFile.replace(baseDir, '')});
+    b2.bundle(function (err, src) {
+        if (err) return t.fail(err);
+        splitSrc = src;
+        pending--;
+        done();
+    });
+
+    function done() {
+        if (pending !== 0) {
+            return;
+        }
+        vm.runInNewContext(splitSrc + mainSrc, { t: t });
+    }
+});

test/external_basedir/main.js

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+t.equal(require('./x'), 'foo');
+t.equal(require('./y'), 'bar');

test/external_basedir/x.js

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+module.exports = 'foo';

test/external_basedir/y.js

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+module.exports = 'bar';