[Fuchsia] Enable TestLauncher bot-mode for all test-suite runs.

Add pass-through for --test-launcher-bot-mode and set it via mb.py for
all non-script steps that we run on Fuchsia.  The main difference this
makes is to enable the TestLauncher's default bot-mode retry-limit.

Bug: 911160
Change-Id: I036c0d466c37dfdcd64c7faebdfc7d25ee6a00a5
Reviewed-on: https://chromium-review.googlesource.com/c/1358636
Commit-Queue: Wez <[email protected]>
Reviewed-by: Kevin Marshall <[email protected]>
Reviewed-by: Erik Chen <[email protected]>
Reviewed-by: John Budorick <[email protected]>
Cr-Commit-Position: refs/heads/master@{#613360}
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index d669146..e2425b0 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -1112,6 +1112,7 @@
       cmdline = [
           '../../testing/test_env.py',
           os.path.join('bin', 'run_%s' % target),
+          '--test-launcher-bot-mode',
       ]
     elif is_simplechrome and test_type != 'script':
       cmdline = [