Disable DoNotMock Error Prone check

Recent Bazel upgrades Error Prone to major version 2.4.0, that is much
stricter now. In addition, starting from guava 29.0-jre, Cache class was
annotated with DoNotMock Error Prone annotation: [1].

Disable the bug pattern, because without using the mock as advertized by
the check would break the corresponding mockito verifications.

[1] https://github.com/google/guava/commit/4eea0f79c27864039a653ec4ca09ef7585e439fc

Change-Id: Ia118ef9eb0c0adb1480488a7b97a0d77a7b31322
diff --git a/BUILD b/BUILD
index 6b8bfbe..3be2354 100644
--- a/BUILD
+++ b/BUILD
@@ -22,6 +22,7 @@
 junit_tests(
     name = "websession_flatfile_tests",
     srcs = glob(["src/test/java/**/*.java"]),
+    javacopts = ["-Xep:DoNotMock:OFF"],
     resources = glob(["src/test/resources/**/*"]),
     tags = ["websession-flatfile"],
     deps = [