fix(ci): raise mocha timeout for the integration suite

[IzaakGough]

• Run the integration suite with a 20s mocha timeout. The Firestore test starts writing less than a second after the emulator comes up, and on a slow runner that first round trip crosses mocha's 2000ms default, so the job fails on timing rather than on anything in the change under test.
• Left the unit suite on the default. It has no external process to wait on, and a low timeout there is still useful.

Same commit passing on one run and timing out on another is what prompted this: run 36005088606 (push) went green while 36005116781 attempt 1 (pull_request) failed on Timeout of 2000ms exceeded. Verified locally that the suite passes with the flag, and that a deliberate 3s test fails without it and passes with it.

[gemini-code-assist]

Code Review

This pull request updates the integrationTest script in package.json to include a 20-second timeout (--timeout 20000) for the Mocha integration tests. There are no review comments, and I have no feedback to provide.