tests/run-multitests.py: Add shortcuts for local Python instances.

One can now use `-i micropython` and `-i cpython` to add instances using
the `MICROPYTHON` and `CPYTHON3` variables (which can be overridden by env
vars).

tests/run-multitests.py

@@ -403,6 +403,10 @@ def main():
     for i in cmd_args.instance:
         if i.startswith("exec:"):
             instances_test.append(PyInstanceSubProcess([i[len("exec:") :]]))
+        elif i == "micropython":
+            instances_test.append(PyInstanceSubProcess([MICROPYTHON]))
+        elif i == "cpython":
+            instances_test.append(PyInstanceSubProcess([CPYTHON3]))
         elif i.startswith("pyb:"):
             instances_test.append(PyInstancePyboard(i[len("pyb:") :]))
         else: