[mobly] Exit non-zero on signals.TestAbortAll

Per https://github.com/google/mobly/issues/960 Mobly incorrectly exits
either 0 or 1 when a test raises signals.TestAbortAll -- depending on
whether any previous tests have failed or not. Instead of exiting 1 in
the case of all tests being aborted, like any other failure, exit with
a special code.

There aren't a lot of standard codes in use, but some internal google
docs suggest 40 for generic retryable errors. So use that.

Bug: b/425675837
Change-Id: Iae58f6c896df2cc3d93ca9c51a84513dcd82db34
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/google/mobly/+/1312367
Reviewed-by: Prashanth Swaminathan <prashanthsw@google.com>

mobly/test_runner.py

diff --git a/mobly/test_runner.py b/mobly/test_runner.py
index b32f5b0..6b4a2bc 100644
--- a/mobly/test_runner.py
+++ b/mobly/test_runner.py
@@ -32,6 +32,9 @@
   pass
 
 
+EXIT_ABORT_ALL = 40
+
+
 def main(argv=None):
   """Execute the test class in a test module.
 
@@ -79,7 +82,7 @@
         runner.run()
         ok = runner.results.is_all_pass and ok
       except signals.TestAbortAll:
-        pass
+        sys.exit(EXIT_ABORT_ALL)  # Exit now, without running any more tests.
       except Exception:
         logging.exception('Exception when executing %s.', config.testbed_name)
         ok = False