Add downstream tests

Lots of packages depend on ClimaCore and it is hard to predict the
ripple effects of small changes. This commit adds tests for the main
downstream repositories that depend on ClimaCore. Passing these tests
are not required to merge PRs, but can be a valuable piece of information.

.github/workflows/Downstream.yml

@@ -0,0 +1,40 @@
+name: Downstream
+on:
+  push:
+    branches:
+      - main
+    tags: '*'
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: downstream ${{ matrix.package }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        package:
+          - 'ClimaAtmos.jl'
+          - 'ClimaCoupler.jl'
+          - 'ClimaDiagnostics.jl'
+          - 'ClimaLand.jl'
+          - 'ClimaTimeSteppers.jl'
+    steps:
+      - uses: actions/checkout@v4
+      - uses: julia-actions/setup-julia@v1
+        with:
+          version: '1.10'
+      - uses: julia-actions/cache@v2
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: actions/checkout@v4
+        with:
+          repository: 'CliMA/${{ matrix.package }}'
+          path: ${{ matrix.package }}
+      - run: |
+          julia --color=yes --project=${{ matrix.package }} -e 'using Pkg; Pkg.instantiate()'
+          julia --color=yes --project=${{ matrix.package }} -e 'using Pkg; Pkg.develop(; path = ".")'
+          julia --color=yes --project=${{ matrix.package }} -e 'using Pkg; Pkg.test()'