[test] Correctly deduce test-driver arguments on bots using GN

This makes auto-detect-build-configs work on bots (i.e.
if --buildbot was passed).

BUG=v8:5533
CQ_INCLUDE_TRYBOTS=master.tryserver.v8:v8_linux64_msan_rel,v8_linux64_tsan_rel,v8_linux_arm64_dbg,v8_linux_gc_stress_dbg,v8_linux_noi18n_rel_ng,v8_linux_nosnap_rel

Review-Url: https://chromiumcodereview.appspot.com/2431653002
Cr-Commit-Position: refs/heads/master@{#40424}

tools/run-tests.py

@@ -452,8 +452,13 @@ def ProcessOptions(options):
       print(">>> Latest GN build found is %s" % latest_config)
       options.outdir = os.path.join(DEFAULT_OUT_GN, latest_config)
 
-  build_config_path = os.path.join(
-      BASE_DIR, options.outdir, "v8_build_config.json")
+  if options.buildbot:
+    build_config_path = os.path.join(
+        BASE_DIR, options.outdir, options.mode, "v8_build_config.json")
+  else:
+    build_config_path = os.path.join(
+        BASE_DIR, options.outdir, "v8_build_config.json")
+
   if os.path.exists(build_config_path):
     try:
       with open(build_config_path) as f:
@@ -464,6 +469,10 @@ def ProcessOptions(options):
       return False
     options.auto_detect = True
 
+    # In auto-detect mode the outdir is always where we found the build config.
+    # This ensures that we'll also take the build products from there.
+    options.outdir = os.path.dirname(build_config_path)
+
     options.arch_and_mode = None
     options.arch = build_config["v8_target_cpu"]
     if options.arch == 'x86':
@@ -711,15 +720,15 @@ def Execute(arch, mode, args, options, suites):
 
   shell_dir = options.shell_dir
   if not shell_dir:
-    if options.buildbot:
+    if options.auto_detect:
+      # If an output dir with a build was passed, test directly in that
+      # directory.
+      shell_dir = os.path.join(BASE_DIR, options.outdir)
+    elif options.buildbot:
       # TODO(machenbach): Get rid of different output folder location on
       # buildbot. Currently this is capitalized Release and Debug.
       shell_dir = os.path.join(BASE_DIR, options.outdir, mode)
       mode = BuildbotToV8Mode(mode)
-    elif options.auto_detect:
-      # If an output dir with a build was passed, test directly in that
-      # directory.
-      shell_dir = os.path.join(BASE_DIR, options.outdir)
     else:
       shell_dir = os.path.join(
           BASE_DIR,