test(is): add is function tests

src/__tests__/is.spec.ts

@@ -0,0 +1,16 @@
+import is from '../is';
+
+describe('is', () => {
+  test('should only return true if all specified values are objects', () => {
+    expect(is({})).toBe(true);
+    expect(is('1')).toBe(false);
+    expect(is(1)).toBe(false);
+    expect(is(false)).toBe(false);
+    expect(is(() => {})).toBe(false);
+  });
+
+  test('should evaluate all values specified', () => {
+    expect(is({}, {}, {})).toBe(true);
+    expect(is({}, {}, 'b')).toBe(false);
+  });
+});