Reorganize how tb infrstructure selects toolchains

tests/conftest.py

@@ -0,0 +1,25 @@
+def pytest_addoption(parser):
+    parser.addoption(
+        "--sim-tool",
+        choices=["questa", "xilinx", "stub", "skip", "auto"],
+        default="auto",
+        help="""
+        Select the simulator to use.
+
+        stub: run the testcase using a no-op simulator stub
+        skip: skip all the simulation tests
+        auto: choose the best simulator based on what is installed
+        """
+    )
+
+    parser.addoption(
+        "--synth-tool",
+        choices=["vivado", "skip", "auto"],
+        default="auto",
+        help="""
+        Select the synthesis tool to use.
+
+        skip: skip all the simulation tests
+        auto: choose the best tool based on what is installed
+        """
+    )