test: add test for fs shim

tests/spec/shims/fs.spec.js

@@ -0,0 +1,44 @@
+'use strict';
+const expect = require('chai').expect;
+const utils = require('../../lib/test-utils');
+const fs = utils.shimIndexedDB(() => require('../../../shims/fs').default);
+
+describe.only('fs shim', () => {
+  it('should be defined', () => {
+    expect(fs).to.not.be.undefined;
+  });
+
+  it('should be an object', () => {
+    expect(typeof fs).to.equal('object');
+  });
+
+  it('should return a function when accessing fs.writeFile', () => {
+    expect(typeof fs.writeFile).to.equal('function');
+  });
+
+  it('should call callback when calling fs.writeFile', (done) => {
+    fs.writeFile('/test.txt', 'test', function(err) {
+      if(err) throw err;
+
+      done();
+    });
+  });
+
+  it('should return an object when accessing fs.promises', () => {
+    expect(typeof fs.promises).to.equal('object');
+  });
+
+  it('should return a function when accessing fs.promises.writeFile', () => {
+    expect(typeof fs.promises.writeFile).to.equal('function');
+  });
+
+  it('should return a promise which resolves when calling fs.promises.writeFile', (done) => {
+    const writeFilePromise = fs.promises.writeFile('/test2.txt', '');
+    expect(writeFilePromise instanceof Promise).to.equal(true);
+    writeFilePromise.then(() => {
+      done();
+    }).catch((err) => {
+      done(err);
+    });
+  });
+});