tests: begin build frontend emulation

setuptools/tests/test_pep_517.py

+import pytest
+import os
+
+# Only test the backend on Python 3
+# because we don't want to require
+# a concurrent.futures backport for testing
+pytest.importorskip('concurrent.futures')
+
+from importlib import import_module
+from concurrent.futures import ProcessPoolExecutor
+
+class BuildBackend(object):
+    """PEP 517 Build Backend"""
+    def __init__(self, cwd=None, env={}, backend_name='setuptools.pep517'):
+        self.cwd = cwd
+        self.env = env
+        self.backend_name = backend_name
+        self.pool = ProcessPoolExecutor()
+
+    def __getattr__(self, name):
+        """Handles aribrary function invokations on the build backend."""
+
+        def method(**kw):
+            return self.pool.submit(
+                BuildBackendCaller(self.cwd, self.env, self.backend_name),
+                (name, kw)).result()
+
+        return method
+
+
+class BuildBackendCaller(object):
+    def __init__(self, cwd, env, backend_name):
+        self.cwd = cwd
+        self.env = env
+        self.backend_name = backend_name
+
+    def __getattr__(self, name):
+        """Handles aribrary function invokations on the build backend."""
+        os.chdir(self.cwd)
+        os.environ.update(self.env)
+        return getattr(import_module(self.backend_name), name)