Fix compile-tests workflow

Moving source files out of the built dir effectively removed the built
dir, causing merge conflicts in the workflow that rebuilt the wasm tests
which were supposed to be in that dir.

Fix by doing a `--strategy ours` merge, then forcibly resetting the tree
state to the main branch, then checking out the built files.  This
ensures that the tree contains precisely the upstream source code, plus
our built files, while keeping the parent tree.

Author: wingo

.github/workflows/compile-tests.yml

@@ -140,7 +140,13 @@ jobs:
         uses: ./.github/actions/git-config
 
       - name: Merge main branch changes
-        run: git merge ${{ github.sha }}
+        run: |
+          git merge --no-commit -Sours ${{ github.sha }}
+          git checkout ${{ github.sha }} .
+          git checkout HEAD tests/assemblyscript/testsuite
+          git checkout HEAD tests/c/testsuite
+          git checkout HEAD tests/rust/testsuite
+          git commit -m "Merge commit ${{ github.sha }} into prod/testsuite-base"
 
       - name: Remove existing binaries
         run: rm -rf tests/${{ matrix.suite }}/testsuite