Various small fixes for nightly infrastructure tests.

- Change the number of iterations for tests from 1 to 3.
- Increase the timeout for image_chromeos.py, so 'cros flash', which
takes a bit longer than cros_image_to_target.sh did, doesn't get killed
just as its finishing.
- Comment out smoothness.top_25 for now, since it's broken and/or
hanging.

BUG=None
TEST=Successfully ran nightly infrastructure tests with these changes.

Change-Id: I77b663b1910fe12ef927afdf15d88a7c1e82d491
Reviewed-on: https://chrome-internal-review.googlesource.com/157419
Reviewed-by: Yunlian Jiang <yunlian@google.com>
Commit-Queue: Caroline Tice <cmtice@google.com>
Tested-by: Caroline Tice <cmtice@google.com>
diff --git a/test_toolchains.py b/test_toolchains.py
index c50185d..d8b750a 100644
--- a/test_toolchains.py
+++ b/test_toolchains.py
@@ -140,7 +140,7 @@
     experiment_tests = """
     benchmark: all_perfv2 {
       suite: telemetry_Crosperf
-      iterations: 1
+      iterations: 3
     }
     """
     with open(experiment_file, "w") as f: