New flag `--incompatible_depset_union` to disable old-style depset merging.

RELNOTES:
  Using `+`, `|` or `.union` on depsets is now deprecated. Please use the new
  constructor instead (see https://docs.bazel.build/versions/master/skylark/depsets.html).
PiperOrigin-RevId: 178262338
diff --git a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
index 714f197..05d5a3e 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
@@ -117,6 +117,7 @@
         "--incompatible_checked_arithmetic=" + rand.nextBoolean(),
         "--incompatible_comprehension_variables_do_not_leak=" + rand.nextBoolean(),
         "--incompatible_depset_is_not_iterable=" + rand.nextBoolean(),
+        "--incompatible_depset_union=" + rand.nextBoolean(),
         "--incompatible_dict_literal_has_no_duplicates=" + rand.nextBoolean(),
         "--incompatible_disallow_dict_plus=" + rand.nextBoolean(),
         "--incompatible_disallow_keyword_only_args=" + rand.nextBoolean(),
@@ -142,6 +143,7 @@
         .incompatibleCheckedArithmetic(rand.nextBoolean())
         .incompatibleComprehensionVariablesDoNotLeak(rand.nextBoolean())
         .incompatibleDepsetIsNotIterable(rand.nextBoolean())
+        .incompatibleDepsetUnion(rand.nextBoolean())
         .incompatibleDictLiteralHasNoDuplicates(rand.nextBoolean())
         .incompatibleDisallowDictPlus(rand.nextBoolean())
         .incompatibleDisallowKeywordOnlyArgs(rand.nextBoolean())