[fuchsia-test-runner] Explicitly target fuchsia-emulator

By explicitly specifying the fuchsia-emulator target, locally running fuchsia-test-runner.py will not fail when there are other targets available.

Author: c6c7

src/ci/docker/scripts/fuchsia-test-runner.py

@@ -618,6 +618,8 @@ def start(self):
         check_call_with_logging(
             [
                 ffx_path,
+                "-t",
+                "fuchsia-emulator",
                 "target",
                 "repository",
                 "register",