semgrep: Add rule to specify tick kwarg on time_machine.travel() calls.

Letting the clock tick without a reason introduces the
possibility of nondeterministic test failures depending on the execution
time. The default value when not specified is tick=True, which makes it
easy to miss.
The rule doesn't prohibit setting tick=True, as perhaps there will be
tests wanting to use that feature on purpose, but such a test should
explicitly set it to make the intent clear.

tools/semgrep-py.yml

@@ -17,6 +17,15 @@ rules:
       include:
         - zerver/views/
 
+  - id: time-machine-travel-specify-tick
+    patterns:
+      - pattern: time_machine.travel(...)
+      - pattern-not: time_machine.travel(..., tick=..., ...)
+    message: |
+      Specify tick kwarg value for time_machine.travel(). Most cases will want to use False.
+    languages: [python]
+    severity: ERROR
+
   - id: limit-message-filter
     patterns:
       - pattern: Message.objects.filter(...)