Fix flaky tests due to Entity name conflicts (#569)

* Fix flaky tests due to Entity name conflicts

Objectify siliently replaces current registration of a given kind
when another class is registered for this kind. There are
several TestObject classes in the current code base, which by
default are all mapped to the same kind.

Tests have only been flaky because impacted tests need to run
in specific orders for failures to happen. Using multiple executors
in Gradle also reduced the likely hood of errors. To reproduce the
problem run the following tests in order (e.g., by putting them in
a test suite):
1. ExportCommitLogDiffActionTest
2. CreateAutoTimestampTest
3. RestoreCommitLogsActionTest

In this PR, we
- Made sure all entities have unique kinds.
- Made all test entities register with AppEngineRule instead of directly
  with ObjectifyService.
- Added code in AppEngineRule to check for re-registrations.
- Added presumit check for forbidden direct registration.

config/presubmits.py

@@ -99,6 +99,15 @@ PRESUBMITS = {
         "System.(out|err).println is only allowed in tools/ packages. Please "
         "use a logger instead.",
 
+    # ObjectifyService.register is restricted to main/ or AppEngineRule.
+    PresubmitCheck(
+        r".*\bObjectifyService\.register", "java", {
+            "/build/", "/generated/", "node_modules/", "src/main/",
+            "AppEngineRule.java"
+        }):
+      "ObjectifyService.register is not allowed in tests. Please use "
+      "AppengineRule.register instead.",
+
     # PostgreSQLContainer instantiation must specify docker tag
     PresubmitCheck(
         r"[\s\S]*new\s+PostgreSQLContainer(<[\s\S]*>)?\(\s*\)[\s\S]*",