Recognize ReachabilityCheckerTestCase as helper

4e196faa1b ("ReachabilityChecker: Default implementation with a
RevWalk", 2019-05-15) added ReachabilityCheckerTestCase but did not add
it as a test helper in the corresponding BUILD file, making tests no
longer runnable with Bazel. Resolve this issue.

Change-Id: Iccc00b0d169dbaa137e130ce2ddd1b7669960b52
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>

org.eclipse.jgit.test/BUILD

@@ -19,6 +19,7 @@ HELPERS = glob(
     "nls/MissingPropertyBundle.java",
     "nls/NoPropertiesBundle.java",
     "nls/NonTranslatedBundle.java",
+    "revwalk/ReachabilityCheckerTestCase.java",
     "revwalk/RevQueueTestCase.java",
     "revwalk/RevWalkTestCase.java",
     "transport/ObjectIdMatcher.java",