Untuck tests out of __tests__ directories, colocate with program files

src/utils/base64.test.ts

@@ -0,0 +1,10 @@
+import * as base64 from './base64';
+
+describe('base64', () => {
+  describe('decode', () => {
+    it('returns a uint8 array', () => {
+      const arr = base64.decode('dGVzdA==');
+      expect(arr).toEqual(new Uint8Array([116, 101, 115, 116]));
+    });
+  });
+});