Reorganize how tb infrstructure selects toolchains

tests/lib/simulators/stub.py

@@ -0,0 +1,17 @@
+from typing import List
+
+from .base import Simulator
+
+class StubSimulator(Simulator):
+    name = "stub"
+
+    @classmethod
+    def is_installed(cls) -> bool:
+        # Always available!
+        return True
+
+    def compile(self) -> None:
+        pass
+
+    def run(self, plusargs: List[str] = None) -> None:
+        pass