Add frontend testing, require node 12 (#15315)

- Add basic frontend unit testing infrastructure using jest in ESM mode
- Rename 'make test' to 'make test-backend'
- Introduce 'make test-frontend' and 'make test' that runs both
- Bump Node.js requirement to v12. v10 will be EOL in less than a month.
- Convert all build-related JS files to ESM.

I opted to run frontend tests run as part of the compliance pipeline because
they complete fast and are not platform-specific like the golang tests.

web_src/js/utils.test.js

@@ -0,0 +1,29 @@
+import {
+  basename, extname, isObject, uniq, stripTags,
+} from './utils.js';
+
+test('basename', () => {
+  expect(basename('/path/to/file.js')).toEqual('file.js');
+  expect(basename('/path/to/file')).toEqual('file');
+  expect(basename('file.js')).toEqual('file.js');
+});
+
+test('extname', () => {
+  expect(extname('/path/to/file.js')).toEqual('.js');
+  expect(extname('/path/')).toEqual('');
+  expect(extname('/path')).toEqual('');
+  expect(extname('file.js')).toEqual('.js');
+});
+
+test('isObject', () => {
+  expect(isObject({})).toBeTrue();
+  expect(isObject([])).toBeFalse();
+});
+
+test('uniq', () => {
+  expect(uniq([1, 1, 1, 2])).toEqual([1, 2]);
+});
+
+test('stripTags', () => {
+  expect(stripTags('<a>test</a>')).toEqual('test');
+});