Reorganize how tb infrstructure selects toolchains

tests/lib/synthesizers/__init__.py

@@ -0,0 +1,30 @@
+from typing import List, Optional, Type
+import functools
+
+from .base import Synthesizer
+from .vivado import Vivado
+
+ALL_SYNTHESIZERS: List[Synthesizer]
+ALL_SYNTHESIZERS = [
+    Vivado,
+]
+
+@functools.lru_cache()
+def get_synthesizer_cls(name: str) -> Optional[Type[Synthesizer]]:
+    if name == "skip":
+        return None
+
+    if name == "auto":
+        # Find the first tool that is installed
+        for synth_cls in ALL_SYNTHESIZERS:
+            if synth_cls.is_installed():
+                return synth_cls
+        raise ValueError("Could not find any installed synthesis tools")
+
+    # Look up which explicit synth tool name was specified
+    for synth_cls in ALL_SYNTHESIZERS:
+        if synth_cls.name == name:
+            if not synth_cls.is_installed():
+                raise ValueError("Synthesis tool '%s' is not installed" % synth_cls.name)
+            return synth_cls
+    raise RuntimeError