test(firefox): disable BigInt tests for Firefox (#4849)

Juggler doesn't yet support BigInt transferring.

Author: aslushnikov

Diff for: test/evaluation.spec.js

@@ -35,7 +35,7 @@ module.exports.addTests = function({testRunner, expect}) {
       const result = await page.evaluate(() => 7 * 3);
       expect(result).toBe(21);
     });
-    (bigint ? it : xit)('should transfer BigInt', async({page, server}) => {
+    (bigint ? it_fails_ffox : xit)('should transfer BigInt', async({page, server}) => {
       const result = await page.evaluate(a => a, BigInt(42));
       expect(result).toBe(BigInt(42));
     });
@@ -143,7 +143,7 @@ module.exports.addTests = function({testRunner, expect}) {
       expect(result).not.toBe(object);
       expect(result).toEqual(object);
     });
-    (bigint ? it : xit)('should return BigInt', async({page, server}) => {
+    (bigint ? it_fails_ffox : xit)('should return BigInt', async({page, server}) => {
       const result = await page.evaluate(() => BigInt(42));
       expect(result).toBe(BigInt(42));
     });