[GHA] Add GitHub Action to run the tests for llvm-zorg itself (#282)

This adds a GitHub action to at least run the very minimal lit tests we
have for zorg itself. With appropriate repository settings it should
provide pass/fail for both PRs and commits.

The GH Actions in the main llvm-repos tend to be gated on `if:
github.repository_owner == 'llvm'`, which I've copied across here.

There's a question mark about how to handle
`test/jenkins/test_build.py`. I've marked it XFAIL as a command-line
`lit` option, as the test ends up trying to call `xcrun` which has no
hope of succeeding on Linux. But perhaps the test is fixable, or it
would be better to mark the test itself XFAIL. Either way, I think the
GH Action in the PR is definitely an improvement over the status quo.

.github/workflows/lit-tests.yml

@@ -0,0 +1,22 @@
+name: llvm-zorg testing
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  check_zorg:
+    name: llvm-zorg lit tests
+    if: github.repository_owner == 'llvm'
+    runs-on: ubuntu-24.04
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install dependencies
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y llvm-18-tools buildbot
+        sudo ln -s /usr/lib/llvm-18/build/utils/lit/lit.py /usr/bin/lit
+        sudo ln -s /usr/bin/FileCheck-18 /usr/bin/FileCheck
+    - name: Run lit tests
+      run: /usr/lib/llvm-18/build/utils/lit/lit.py -v --xfail jenkins/test_build.py test