Skip parser-only expression checks on Firefox

Firefox 67 on BrowserStack disconnects while running the full unit suite
after this PR adds the generated-code compatibility checks. The runtime
expression tests still pass in Firefox; the problematic block is the
Acorn parser loop that validates generated JavaScript syntax against the
oldest supported ECMAScript target.

Keep that compatibility coverage for the other BrowserStack browsers,
but mark it pending on Firefox so Firefox still exercises the debugger
runtime behavior without spending time on parser-only coverage.

Author: watson

packages/debugger/src/domain/expression.spec.ts

@@ -1,4 +1,5 @@
 import * as acorn from 'acorn'
+import { isFirefox } from '@datadog/browser-core/test'
 import {
   literals,
   references,
@@ -158,6 +159,12 @@ describe('Expression language', () => {
 // bypassing TypeScript compilation and webpack transpilation. This test catches usage of syntax
 // not supported by older browsers (e.g., optional chaining ?., optional catch binding, nullish coalescing ??).
 describe('browser compatibility of generated code', () => {
+  beforeEach(() => {
+    if (isFirefox()) {
+      pending('Browser compatibility is parser-only coverage and is too expensive for Firefox on BrowserStack')
+    }
+  })
+
   function assertECMAVersionCompatible(code: string, params: string[] = []) {
     const functionCode = `function f(${params.join(', ')}) { ${code} }`
     try {