casper test: Refine --loop mode.

We added a really nice feature recently,
called `--interactive`, which lets you loop
through Casper tests without having to restart
it every time.

I am renaming it to `--loop` and adding a few
features:

- The first loop will just run without you having
  to tell it to start.  (This means you don't have
  to sit there while waiting for webpack to finish
  and for the server to start, just to launch
  the tests again.)

- You specify how many loops you want to run,
  which means in the success case, it won't
  just keep going forever--it will eventually
  stop, giving you an opportunity to refine
  the test further without re-launching.

frontend_tests/run-casper

@@ -21,20 +21,27 @@ os.environ["PHANTOMJS_EXECUTABLE"] = os.path.join(ZULIP_PATH, "node_modules/.bin
 os.environ.pop("http_proxy", "")
 os.environ.pop("https_proxy", "")
 
-usage = """test-js-with-casper [options]
+usage = """
+
     test-js-with-casper # Run all test files
     test-js-with-casper 09-navigation.js # Run a single test file
     test-js-with-casper 09 # Run a single test file 09-navigation.js
     test-js-with-casper 01-login.js 03-narrow.js # Run a few test files
-    test-js-with-casper 01 03 # Run a few test files, 01-login.js and 03-narrow.js here"""
-parser = argparse.ArgumentParser(usage)
+    test-js-with-casper 01 03 # Run a few test files, 01-login.js and 03-narrow.js here
+
+    Using loops:
+
+        test-js-with-capser --loop 5 07 # run 5 loops of test 07
+---
+"""
+parser = argparse.ArgumentParser(usage=usage)
 
 parser.add_argument('--skip-flaky-tests', dest='skip_flaky',
                     action="store_true",
                     default=False, help='Skip flaky tests')
-parser.add_argument('--interactive', dest='interactive',
-                    action="store_true",
-                    default=False, help='Run tests interactively')
+parser.add_argument('--loop', dest='loop', nargs=1,
+                    action="store", type=int,
+                    default=None, help='Run tests in a loop.')
 parser.add_argument('--force', dest='force',
                     action="store_true",
                     default=False, help='Run tests despite possible problems.')
@@ -98,6 +105,16 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
     if options.skip_flaky:
         test_files = [fn for fn in test_files if '10-admin' not in fn]
 
+    if options.loop:
+        loop_cnt = options.loop[0]
+        print('\n\nWe will use loop mode for these tests:\n')
+        for test_file in test_files:
+            print('    ' + os.path.basename(test_file))
+        print('\nnumber of loops: {}\n'.format(loop_cnt))
+        print()
+    else:
+        loop_cnt = None
+
     remote_debug = []  # type: List[str]
     if options.remote_debug:
         remote_debug = ["--remote-debugger-port=7777", "--remote-debugger-autorun=yes"]
@@ -121,6 +138,27 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
                 return ret
         return 0
 
+    def run_loops(loop_cnt: int) -> None:
+        while True:
+            for trial in range(1, loop_cnt + 1):
+                print('\n\n\nSTARTING TRIAL {} / {}\n'.format(trial, loop_cnt))
+                ret = run_tests()
+                if ret == 0:
+                    print('`\n\nSUCCESS! trial #{}\n\n'.format(trial))
+                else:
+                    print('\n\nFAIL! trial #{}\n'.format(trial))
+                    break
+
+            while True:
+                response = input('Press "q" to quit or enter number of loops: ')
+                if response == 'q':
+                    return
+                try:
+                    loop_cnt = int(response)
+                    break
+                except ValueError:
+                    continue
+
     with test_server_running(options.force, external_host):
         # Important: do next things inside the `with` block, when Django
         #            will be pointing at the test database.
@@ -128,15 +166,10 @@ def run_tests(files: Iterable[str], external_host: str) -> None:
         subprocess.check_call('tools/setup/generate-test-credentials')
 
         # RUN THE TESTS!!!
-        if options.interactive:
-            response = input('Press Enter to run tests, "q" to quit: ')
-            ret = 1
-            while response != 'q':
-                ret = run_tests()
-                if ret != 0:
-                    response = input('Tests failed. Press Enter to re-run tests, "q" to quit: ')
+        if loop_cnt:
+            run_loops(loop_cnt)
+            ret = 0
         else:
-            ret = 1
             ret = run_tests()
 
     if ret != 0: