gateware: don't run tests if there is no migen.

This allows us to skip testing gateware on Windows.

artiq/gateware/test/__init__.py

@@ -0,0 +1,7 @@
+import unittest
+
+try:
+    import migen
+except ImportError:
+    def load_tests(*args, **kwargs):
+        raise unittest.SkipTest("migen unavailable")