fix: correctly order arguments in Jest diff util (#1245)

Some custom matchers like `@testing-library/jest-dom` use
`this.utils.diff` for the output. In Jest the order of arguments in that
util is "expected", then "received", so we're fixing the order of those
arguments here to match Jest's implementation.

packages/vitest/src/integrations/chai/jest-matcher-utils.ts

@@ -170,5 +170,5 @@ export interface DiffOptions {
 // TODO: do something with options
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
 export function diff(a: any, b: any, options?: DiffOptions) {
-  return unifiedDiff(stringify(a), stringify(b))
+  return unifiedDiff(stringify(b), stringify(a))
 }

test/core/test/jest-matcher-utils.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest'
+
+describe('jest-matcher-utils', () => {
+  expect.extend({
+    toBeJestEqual(received: any, expected: any) {
+      return {
+        message: () => this.utils.diff(expected, received),
+        pass: received === expected,
+      }
+    },
+  })
+
+  it('diff', () => {
+    expect(() => {
+      // @ts-expect-error "toBeJestEqual" is a custom matcher we just created
+      expect('a').toBeJestEqual('b')
+    }).toThrowError(/Expected.*"b".*Received.*"a"/ms)
+  })
+})