bisect-kit: verify autotest prebuilt is installed

BUG=None
TEST=run "eval_cros_autotest.py --prebuilt" without running switch_autotest_prebuilt.py first

Change-Id: I63303a42dfc3f2fd2dce955367c700586039949f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2651776
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
diff --git a/eval_cros_autotest.py b/eval_cros_autotest.py
index 57e4b02..ed44e3c 100755
--- a/eval_cros_autotest.py
+++ b/eval_cros_autotest.py
@@ -334,6 +334,13 @@
     if not opts.chrome_root:
       logger.error('--chrome_root is mandatory for telemetry tests')
       return FATAL
+  if opts.prebuilt:
+    autotest_dir = os.path.join(opts.chromeos_root,
+                                cros_util.prebuilt_autotest_dir)
+    if not os.path.exists(autotest_dir):
+      parser.error('--prebuilt: no autotest prebuilt installed (%s); '
+                   'please run switch_autotest_prebuilt.py first' %
+                   autotest_dir)
 
   try:
     prepare_to_run_test(opts)