testing: Run in parallel mode by default.

This commit changes the backend testing framework to run
in parallel mode which is same as --processes=4. If --coverage
is supplied, we enforce serial mode, --processes=1, because
coverage is not compatible with parallel mode at the moment.

docs/testing-with-django.md

@@ -24,10 +24,11 @@ or just done things that are kind of unique in Zulip.
 ## Running tests
 
 Our tests live in `zerver/tests/`. You can run them with
-`./tools/test-backend`. It generally takes about a minute to run
-the entire test suite.  When you are in iterative mode, you
-can run individual tests or individual modules, following the
-dotted.test.name convention below:
+`./tools/test-backend`. The tests run in parallel using multiple
+threads in your development environment, and can finish in under 30s
+on a fast machine.  When you are in iterative mode, you can run
+individual tests or individual modules, following the dotted.test.name
+convention below:
 
     cd /srv/zulip
     ./tools/test-backend zerver.tests.test_queue_worker.WorkerTest

tools/test-backend

@@ -144,7 +144,8 @@ if __name__ == "__main__":
                       dest="fatal_errors", help="Continue past test failures to run all tests")
     parser.add_option('--coverage', dest='coverage',
                       action="store_true",
-                      default=False, help='Compute test coverage.')
+                      default=False,
+                      help='Compute test coverage. Enforces processes=1.')
     parser.add_option('--verbose-coverage', dest='verbose_coverage',
                       action="store_true",
                       default=False, help='Enable verbose print of coverage report.')
@@ -160,9 +161,9 @@ if __name__ == "__main__":
                       type="int",
                       callback=allow_positive_int,
                       action='callback',
-                      default=1,
+                      default=4,
                       help='Specify the number of processes to run the '
-                           'tests in. Default is 1.')
+                           'tests in. Default is 4.')
     parser.add_option('--profile', dest='profile',
                       action="store_true",
                       default=False, help='Profile test runtime.')
@@ -195,6 +196,11 @@ if __name__ == "__main__":
                             "test-backend was run.  Implies --nonfatal-errors."))
 
     (options, args) = parser.parse_args()
+    if options.coverage:
+        # Currently coverage doesn't work with parallel mode, so when
+        # coverage parameter is supplied we enfore serial mode.
+        print("Disabling parallel mode because coverage isn't supported.")
+        options.processes = 1
 
     zerver_test_dir = 'zerver/tests/'