Enable linked PR test for pull request tests

[itingliu]

According to https://github.com/swiftlang/github-workflows, to invoke:

To reference a linked PR, add Linked PR: <link to PR> to the PR description, eg.

Linked PR: https://github.com/swiftlang/swift-syntax/pull/2859
// or alternatively
Linked PR: swiftlang/swift-syntax#2859

Enabling cross-PR testing will add about 10s to PR testing time.

[itingliu]

@swift-ci please test