Add basic test case for interpolations

src/codegen.js

···
 
 const assignIndex = (depth) =>
   js`var y${depth} = ${_state}.y` +
-  (_interpolations ? js`, var x${depth} = ${_state}.x;` : ';');
+  (_interpolations ? js`, x${depth} = ${_state}.x;` : ';');
 
 const restoreIndex = (depth) =>
   js`${_state}.y = y${depth}` +
···
   opts = copy(opts);
   opts.capture = capture;
 
-  let group = '';
   if (!opts.length && capture) {
     opts.length = depth;
     return js`

src/core.test.js

···
-import { match } from './core';
+import { parse, match, interpolation } from './core';
 
 const expectToParse = (node, input, result, lastIndex = 0) => {
   const state = { quasis: [input], expressions: [], x: 0, y: 0 };
···
     }
   );
 });
+
+describe('interpolation parsing', () => {
+  const node = match('node')`
+    ${/1/}
+    ${interpolation}
+    ${/3/}
+  `;
+
+  const expected = ['1', 2, '3'];
+  expected.tag = 'node';
+
+  expect(parse(node)`1${2}3`).toEqual(expected);
+  expect(parse(node)`13`).toBe(undefined);
+  expect(parse(node)`13${2}`).toBe(undefined);
+  expect(parse(node)`${2}13`).toBe(undefined);
+});