Telemetry: Use the leftmost argument to find the test to parse flags.

Previously it would check all of the arguments, and use the last test found.
This is causes problems as different tests can be used to create the parser and
accepts the parsed flags.

SHERIFFS: If errors related to parsing flags for run_measurement, run_benchmark
or run_gpu_test.py appear, this is likely the cause. It is safe to revert.

BUG=413334

Review URL: https://codereview.chromium.org/582793002

Cr-Commit-Position: refs/heads/master@{#295738}

tools/telemetry/telemetry/test_runner.py

@@ -111,15 +111,23 @@ class Run(command_line.OptparseCommand):
   def AddCommandLineArgs(cls, parser):
     benchmark.AddCommandLineArgs(parser)
 
+    # TODO(jbroman): This compensates for the fact that the "run" command
+    # may or may not be given on the command line. It's inelegant.
+    if len(sys.argv) > 2 and 'run'.startswith(sys.argv[1]):
+      test_name = sys.argv[2]
+    elif len(sys.argv) > 1:
+      test_name = sys.argv[1]
+    else:
+      # No arguments, so no more flags needed.
+      return
+
     # Allow tests to add their own command line options.
-    matching_tests = []
-    for arg in sys.argv[1:]:
-      matching_tests += _MatchTestName(arg)
+    matching_tests = _MatchTestName(test_name)
 
     if matching_tests:
       # TODO(dtu): After move to argparse, add command-line args for all tests
       # to subparser. Using subparsers will avoid duplicate arguments.
-      matching_test = matching_tests.pop()
+      matching_test = matching_tests[0]
       matching_test.AddCommandLineArgs(parser)
       # The test's options override the defaults!
       matching_test.SetArgumentDefaults(parser)
@@ -246,6 +254,8 @@ def _MatchTestName(input_test_name, exact_matches=True):
     for test_class in _Tests():
       if exact_match == test_class.Name():
         return [test_class]
+    # TODO(jbroman): [] is the correct result when no exact match is found.
+    # However, some bots rely on this falling through right now.
 
   # Fuzzy matching.
   return [test_class for test_class in _Tests()