Add jasmine tests for filtered search dropdown manager

spec/javascripts/filtered_search/filtered_search_dropdown_manager_spec.js.es6

+//= require filtered_search/filtered_search_tokenizer
+//= require filtered_search/filtered_search_dropdown_manager
+
+(() => {
+  describe('Filtered Search Dropdown Manager', () => {
+    describe('addWordToInput', () => {
+      describe('add word and when lastToken is an empty object', () => {
+        function getInput() {
+          return document.querySelector('.filtered-search');
+        }
+
+        beforeEach(() => {
+          spyOn(gl.FilteredSearchTokenizer, 'processTokens')
+            .and.callFake(query => ({
+                lastToken: {}
+              })
+            );
+
+          const input = document.createElement('input');
+          input.classList.add('filtered-search');
+          document.body.appendChild(input);
+
+          expect(input.value).toBe('');
+        });
+
+        afterEach(() => {
+          document.querySelector('.filtered-search').outerHTML = '';
+        });
+
+        it('should add word', () => {
+          gl.FilteredSearchDropdownManager.addWordToInput('firstWord');
+          expect(getInput().value).toBe('firstWord');
+        });
+
+        it('should not add space before first word', () => {
+          gl.FilteredSearchDropdownManager.addWordToInput('firstWord', true);
+          expect(getInput().value).toBe('firstWord');
+        });
+
+        it('should not add space before second word by default', () => {
+          gl.FilteredSearchDropdownManager.addWordToInput('firstWord');
+          expect(getInput().value).toBe('firstWord');
+          gl.FilteredSearchDropdownManager.addWordToInput('secondWord');
+          expect(getInput().value).toBe('firstWordsecondWord');
+        });
+
+        it('should add space before new word when addSpace is passed', () => {
+          expect(getInput().value).toBe('');
+          gl.FilteredSearchDropdownManager.addWordToInput('firstWord');
+          expect(getInput().value).toBe('firstWord');
+          gl.FilteredSearchDropdownManager.addWordToInput('secondWord', true);
+          expect(getInput().value).toBe('firstWord secondWord');
+        });
+      });
+    });
+  });
+})();