Fix CI: run tests from /tmp to use installed package

The source diffcp/ directory was shadowing the installed package,
causing 'ModuleNotFoundError: No module named diffcp._diffcp'.

By running pytest from /tmp with the full path to tests, Python
imports from site-packages where the compiled _diffcp extension
is properly installed.

Author: SteveDiamond

.github/workflows/build.yml

@@ -32,7 +32,8 @@ jobs:
           pip install -v .[test]
 
       - name: Run tests
-        run: pytest tests/ -v
+        run: pytest $GITHUB_WORKSPACE/tests/ -v
+        working-directory: /tmp
 
   build_wheels:
     name: Build wheels on ${{ matrix.os }}